I am using SQL Server 2008 Enterprise with Windows Server 2008 Enterprise. I have a database table called "Book", which has many columns and three columns are important in this question, they are
Author, varchar;
Country, varchar;
Domain, varchar.
I want to write a store procedure with the following logics, but I do not know how to write (because of complex query conditions), appreciate if anyone could write a sample for me?
Input parameter: p_author as varchar, p_country as varchar, and p_domain as varchar
Query conditions:
if p_author is specified from input, then any row whose Author column LIKE %p_author% is satisfied with condition, if p_author is not specified from input every row is satisfied with this condition;
if p_country is specified from input, then any row whose Country column = p_country is satisfied with condition, if p_country is not specified from input every row is satisfied with this condition;
if p_domain is specified from input, then any row whose Domain column LIKE %p_domain% is satisfied, if p_domain is not specified from input every row is satisfied with this condition;
The results I want to return (must met with all following conditions):
records met with either condition 1 or 2;
records must meet with condition 3;
return distinct rows.
For example, records which met with condition 1 and condition 3 are ok to return, and records which met with condition 2 and condition 3 are ok to return.
thanks in advance,
George
Dynamically changing searches based on the given parameters is a complicated subject and doing it one way over another, even with only a very slight difference, can have massive performance implications. The key is to use an index, ignore compact code, ignore worrying about repeating code, you must make a good query execution plan (use an index).
Read this and consider all the methods. Your best method will depend on your parameters, your data, your schema, and your actual usage:
Dynamic Search Conditions in T-SQL by by Erland Sommarskog
The Curse and Blessings of Dynamic SQL by Erland Sommarskog
If you have the proper SQL Server 2008 version (SQL 2008 SP1 CU5 (10.0.2746) and later), you can use this little trick to actually use an index:
There isn't much you can do since you are using LIKE, but if you were using equality, you could add OPTION (RECOMPILE) onto your query, see Erland's article, and SQL Server will resolve the OR from within (Column = #Param+'%' OR #Param='') AND ... before the query plan is created based on the run-time values of the local variables, and an index can be used if you weren't using LIKE.
If I understand correctly, the following should work:
SELECT *
FROM Books
WHERE (
((Author LIKE '%' + #p_author + '%' OR #p_author = '') OR
(Country LIKE '%' + #p_country + '%' OR #p_country = ''))
AND (#p_author <> '' OR #p_country <> '')
) AND
(Domain LIKE '%' + #p_domain + '%' OR '%' #p_domain = '')
Related
Why would someone use WHERE 1=1 AND <conditions> in a SQL clause (Either SQL obtained through concatenated strings, either view definition)
I've seen somewhere that this would be used to protect against SQL Injection, but it seems very weird.
If there is injection WHERE 1 = 1 AND injected OR 1=1 would have the same result as injected OR 1=1.
Later edit: What about the usage in a view definition?
Thank you for your answers.
Still,
I don't understand why would someone use this construction for defining a view, or use it inside a stored procedure.
Take this for example:
CREATE VIEW vTest AS
SELECT FROM Table WHERE 1=1 AND table.Field=Value
If the list of conditions is not known at compile time and is instead built at run time, you don't have to worry about whether you have one or more than one condition. You can generate them all like:
and <condition>
and concatenate them all together. With the 1=1 at the start, the initial and has something to associate with.
I've never seen this used for any kind of injection protection, as you say it doesn't seem like it would help much. I have seen it used as an implementation convenience. The SQL query engine will end up ignoring the 1=1 so it should have no performance impact.
Just adding a example code to Greg's answer:
dim sqlstmt as new StringBuilder
sqlstmt.add("SELECT * FROM Products")
sqlstmt.add(" WHERE 1=1")
''// From now on you don't have to worry if you must
''// append AND or WHERE because you know the WHERE is there
If ProductCategoryID <> 0 then
sqlstmt.AppendFormat(" AND ProductCategoryID = {0}", trim(ProductCategoryID))
end if
If MinimunPrice > 0 then
sqlstmt.AppendFormat(" AND Price >= {0}", trim(MinimunPrice))
end if
I've seen it used when the number of conditions can be variable.
You can concatenate conditions using an " AND " string. Then, instead of counting the number of conditions you're passing in, you place a "WHERE 1=1" at the end of your stock SQL statement and throw on the concatenated conditions.
Basically, it saves you having to do a test for conditions and then add a "WHERE" string before them.
Seems like a lazy way to always know that your WHERE clause is already defined and allow you to keep adding conditions without having to check if it is the first one.
Indirectly Relevant: when 1=2 is used:
CREATE TABLE New_table_name
as
select *
FROM Old_table_name
WHERE 1 = 2;
this will create a new table with same schema as old table. (Very handy if you want to load some data for compares)
I found this pattern useful when I'm testing or double checking things on the database, so I can very quickly comment other conditions:
CREATE VIEW vTest AS
SELECT FROM Table WHERE 1=1
AND Table.Field=Value
AND Table.IsValid=true
turns into:
CREATE VIEW vTest AS
SELECT FROM Table WHERE 1=1
--AND Table.Field=Value
--AND Table.IsValid=true
1 = 1 expression is commonly used in generated sql code. This expression can simplify sql generating code reducing number of conditional statements.
Actually, I've seen this sort of thing used in BIRT reports. The query passed to the BIRT runtime is of the form:
select a,b,c from t where a = ?
and the '?' is replaced at runtime by an actual parameter value selected from a drop-down box. The choices in the drop-down are given by:
select distinct a from t
union all
select '*' from sysibm.sysdummy1
so that you get all possible values plus "*". If the user selects "*" from the drop down box (meaning all values of a should be selected), the query has to be modified (by Javascript) before being run.
Since the "?" is a positional parameter and MUST remain there for other things to work, the Javascript modifies the query to be:
select a,b,c from t where ((a = ?) or (1==1))
That basically removes the effect of the where clause while still leaving the positional parameter in place.
I've also seen the AND case used by lazy coders whilst dynamically creating an SQL query.
Say you have to dynamically create a query that starts with select * from t and checks:
the name is Bob; and
the salary is > $20,000
some people would add the first with a WHERE and subsequent ones with an AND thus:
select * from t where name = 'Bob' and salary > 20000
Lazy programmers (and that's not necessarily a bad trait) wouldn't distinguish between the added conditions, they'd start with select * from t where 1=1 and just add AND clauses after that.
select * from t where 1=1 and name = 'Bob' and salary > 20000
where 1=0, This is done to check if the table exists. Don't know why 1=1 is used.
While I can see that 1=1 would be useful for generated SQL, a technique I use in PHP is to create an array of clauses and then do
implode (" AND ", $clauses);
thus avoiding the problem of having a leading or trailing AND. Obviously this is only useful if you know that you are going to have at least one clause!
Here's a closely related example: using a SQL MERGE statement to update the target tabled using all values from the source table where there is no common attribute on which to join on e.g.
MERGE INTO Circles
USING
(
SELECT pi
FROM Constants
) AS SourceTable
ON 1 = 1
WHEN MATCHED THEN
UPDATE
SET circumference = 2 * SourceTable.pi * radius;
If you came here searching for WHERE 1, note that WHERE 1 and WHERE 1=1 are identical. WHERE 1 is used rarely because some database systems reject it considering WHERE 1 not really being boolean.
Why would someone use WHERE 1=1 AND <proper conditions>
I've seen homespun frameworks do stuff like this (blush), as this allows lazy parsing practices to be applied to both the WHERE and AND Sql keywords.
For example (I'm using C# as an example here), consider the conditional parsing of the following predicates in a Sql query string builder:
var sqlQuery = "SELECT * FROM FOOS WHERE 1 = 1"
if (shouldFilterForBars)
{
sqlQuery = sqlQuery + " AND Bars > 3";
}
if (shouldFilterForBaz)
{
sqlQuery = sqlQuery + " AND Baz < 12";
}
The "benefit" of WHERE 1 = 1 means that no special code is needed:
For AND - whether zero, one or both predicates (Bars and Baz's) should be applied, which would determine whether the first AND is required. Since we already have at least one predicate with the 1 = 1, it means AND is always OK.
For no predicates at all - In the case where there are ZERO predicates, then the WHERE must be dropped. But again, we can be lazy, because we are again guarantee of at least one predicate.
This is obviously a bad idea and would recommend using an established data access framework or ORM for parsing optional and conditional predicates in this way.
Having review all the answers i decided to perform some experiment like
SELECT
*
FROM MyTable
WHERE 1=1
Then i checked with other numbers
WHERE 2=2
WHERE 10=10
WHERE 99=99
ect
Having done all the checks, the query run town is the same. even without the where clause. I am not a fan of the syntax
This is useful in a case where you have to use dynamic query in which in where
clause you have to append some filter options. Like if you include options 0 for status is inactive, 1 for active. Based from the options, there is only two available options(0 and 1) but if you want to display All records, it is handy to include in where close 1=1.
See below sample:
Declare #SearchValue varchar(8)
Declare #SQLQuery varchar(max) = '
Select [FirstName]
,[LastName]
,[MiddleName]
,[BirthDate]
,Case
when [Status] = 0 then ''Inactive''
when [Status] = 1 then ''Active''
end as [Status]'
Declare #SearchOption nvarchar(100)
If (#SearchValue = 'Active')
Begin
Set #SearchOption = ' Where a.[Status] = 1'
End
If (#SearchValue = 'Inactive')
Begin
Set #SearchOption = ' Where a.[Status] = 0'
End
If (#SearchValue = 'All')
Begin
Set #SearchOption = ' Where 1=1'
End
Set #SQLQuery = #SQLQuery + #SearchOption
Exec(#SQLQuery);
Saw this in production code and asked seniors for help.
Their answer:
-We use 1=1 so when we have to add a new condition we can just type
and <condition>
and get on with it.
I do this usually when I am building dynamic SQL for a report which has many dropdown values a user can select. Since the user may or may not select the values from each dropdown, we end up getting a hard time figuring out which condition was the first where clause. So we pad up the query with a where 1=1 in the end and add all where clauses after that.
Something like
select column1, column2 from my table where 1=1 {name} {age};
Then we would build the where clause like this and pass it as a parameter value
string name_whereClause= ddlName.SelectedIndex > 0 ? "AND name ='"+ ddlName.SelectedValue+ "'" : "";
As the where clause selection are unknown to us at runtime, so this helps us a great deal in finding whether to include an 'AND' or 'WHERE'.
Making "where 1=1" the standard for all your queries also makes it trivially easy to validate the sql by replacing it with where 1 = 0, handy when you have batches of commands/files.
Also makes it trivially easy to find the end of the end of the from/join section of any query. Even queries with sub-queries if properly indented.
I first came across this back with ADO and classic asp, the answer i got was: performance.
if you do a straight
Select * from tablename
and pass that in as an sql command/text you will get a noticeable performance increase with the
Where 1=1
added, it was a visible difference. something to do with table headers being returned as soon as the first condition is met, or some other craziness, anyway, it did speed things up.
Using a predicate like 1=1 is a normal hint sometimes used to force the access plan to use or not use an index scan. The reason why this is used is when you are using a multi-nested joined query with many predicates in the where clause where sometimes even using all of the indexes causes the access plan to read each table - a full table scan. This is just 1 of many hints used by DBAs to trick a dbms into using a more efficient path. Just don't throw one in; you need a dba to analyze the query since it doesn't always work.
Here is a use case... however I am not too concerned with the technicalities of why I should or not use 1 = 1.
I am writing a function, using pyodbc to retrieve some data from SQL Server. I was looking for a way to force a filler after the where keyword in my code. This was a great suggestion indeed:
if _where == '': _where = '1=1'
...
...
...
cur.execute(f'select {predicate} from {table_name} where {_where}')
The reason is because I could not implement the keyword 'where' together inside the _where clause variable. So, I think using any dummy condition that evaluates to true would do as a filler.
Why would someone use WHERE 1=1 AND <conditions> in a SQL clause (Either SQL obtained through concatenated strings, either view definition)
I've seen somewhere that this would be used to protect against SQL Injection, but it seems very weird.
If there is injection WHERE 1 = 1 AND injected OR 1=1 would have the same result as injected OR 1=1.
Later edit: What about the usage in a view definition?
Thank you for your answers.
Still,
I don't understand why would someone use this construction for defining a view, or use it inside a stored procedure.
Take this for example:
CREATE VIEW vTest AS
SELECT FROM Table WHERE 1=1 AND table.Field=Value
If the list of conditions is not known at compile time and is instead built at run time, you don't have to worry about whether you have one or more than one condition. You can generate them all like:
and <condition>
and concatenate them all together. With the 1=1 at the start, the initial and has something to associate with.
I've never seen this used for any kind of injection protection, as you say it doesn't seem like it would help much. I have seen it used as an implementation convenience. The SQL query engine will end up ignoring the 1=1 so it should have no performance impact.
Just adding a example code to Greg's answer:
dim sqlstmt as new StringBuilder
sqlstmt.add("SELECT * FROM Products")
sqlstmt.add(" WHERE 1=1")
''// From now on you don't have to worry if you must
''// append AND or WHERE because you know the WHERE is there
If ProductCategoryID <> 0 then
sqlstmt.AppendFormat(" AND ProductCategoryID = {0}", trim(ProductCategoryID))
end if
If MinimunPrice > 0 then
sqlstmt.AppendFormat(" AND Price >= {0}", trim(MinimunPrice))
end if
I've seen it used when the number of conditions can be variable.
You can concatenate conditions using an " AND " string. Then, instead of counting the number of conditions you're passing in, you place a "WHERE 1=1" at the end of your stock SQL statement and throw on the concatenated conditions.
Basically, it saves you having to do a test for conditions and then add a "WHERE" string before them.
Seems like a lazy way to always know that your WHERE clause is already defined and allow you to keep adding conditions without having to check if it is the first one.
Indirectly Relevant: when 1=2 is used:
CREATE TABLE New_table_name
as
select *
FROM Old_table_name
WHERE 1 = 2;
this will create a new table with same schema as old table. (Very handy if you want to load some data for compares)
I found this pattern useful when I'm testing or double checking things on the database, so I can very quickly comment other conditions:
CREATE VIEW vTest AS
SELECT FROM Table WHERE 1=1
AND Table.Field=Value
AND Table.IsValid=true
turns into:
CREATE VIEW vTest AS
SELECT FROM Table WHERE 1=1
--AND Table.Field=Value
--AND Table.IsValid=true
1 = 1 expression is commonly used in generated sql code. This expression can simplify sql generating code reducing number of conditional statements.
Actually, I've seen this sort of thing used in BIRT reports. The query passed to the BIRT runtime is of the form:
select a,b,c from t where a = ?
and the '?' is replaced at runtime by an actual parameter value selected from a drop-down box. The choices in the drop-down are given by:
select distinct a from t
union all
select '*' from sysibm.sysdummy1
so that you get all possible values plus "*". If the user selects "*" from the drop down box (meaning all values of a should be selected), the query has to be modified (by Javascript) before being run.
Since the "?" is a positional parameter and MUST remain there for other things to work, the Javascript modifies the query to be:
select a,b,c from t where ((a = ?) or (1==1))
That basically removes the effect of the where clause while still leaving the positional parameter in place.
I've also seen the AND case used by lazy coders whilst dynamically creating an SQL query.
Say you have to dynamically create a query that starts with select * from t and checks:
the name is Bob; and
the salary is > $20,000
some people would add the first with a WHERE and subsequent ones with an AND thus:
select * from t where name = 'Bob' and salary > 20000
Lazy programmers (and that's not necessarily a bad trait) wouldn't distinguish between the added conditions, they'd start with select * from t where 1=1 and just add AND clauses after that.
select * from t where 1=1 and name = 'Bob' and salary > 20000
where 1=0, This is done to check if the table exists. Don't know why 1=1 is used.
While I can see that 1=1 would be useful for generated SQL, a technique I use in PHP is to create an array of clauses and then do
implode (" AND ", $clauses);
thus avoiding the problem of having a leading or trailing AND. Obviously this is only useful if you know that you are going to have at least one clause!
Here's a closely related example: using a SQL MERGE statement to update the target tabled using all values from the source table where there is no common attribute on which to join on e.g.
MERGE INTO Circles
USING
(
SELECT pi
FROM Constants
) AS SourceTable
ON 1 = 1
WHEN MATCHED THEN
UPDATE
SET circumference = 2 * SourceTable.pi * radius;
If you came here searching for WHERE 1, note that WHERE 1 and WHERE 1=1 are identical. WHERE 1 is used rarely because some database systems reject it considering WHERE 1 not really being boolean.
Why would someone use WHERE 1=1 AND <proper conditions>
I've seen homespun frameworks do stuff like this (blush), as this allows lazy parsing practices to be applied to both the WHERE and AND Sql keywords.
For example (I'm using C# as an example here), consider the conditional parsing of the following predicates in a Sql query string builder:
var sqlQuery = "SELECT * FROM FOOS WHERE 1 = 1"
if (shouldFilterForBars)
{
sqlQuery = sqlQuery + " AND Bars > 3";
}
if (shouldFilterForBaz)
{
sqlQuery = sqlQuery + " AND Baz < 12";
}
The "benefit" of WHERE 1 = 1 means that no special code is needed:
For AND - whether zero, one or both predicates (Bars and Baz's) should be applied, which would determine whether the first AND is required. Since we already have at least one predicate with the 1 = 1, it means AND is always OK.
For no predicates at all - In the case where there are ZERO predicates, then the WHERE must be dropped. But again, we can be lazy, because we are again guarantee of at least one predicate.
This is obviously a bad idea and would recommend using an established data access framework or ORM for parsing optional and conditional predicates in this way.
Having review all the answers i decided to perform some experiment like
SELECT
*
FROM MyTable
WHERE 1=1
Then i checked with other numbers
WHERE 2=2
WHERE 10=10
WHERE 99=99
ect
Having done all the checks, the query run town is the same. even without the where clause. I am not a fan of the syntax
This is useful in a case where you have to use dynamic query in which in where
clause you have to append some filter options. Like if you include options 0 for status is inactive, 1 for active. Based from the options, there is only two available options(0 and 1) but if you want to display All records, it is handy to include in where close 1=1.
See below sample:
Declare #SearchValue varchar(8)
Declare #SQLQuery varchar(max) = '
Select [FirstName]
,[LastName]
,[MiddleName]
,[BirthDate]
,Case
when [Status] = 0 then ''Inactive''
when [Status] = 1 then ''Active''
end as [Status]'
Declare #SearchOption nvarchar(100)
If (#SearchValue = 'Active')
Begin
Set #SearchOption = ' Where a.[Status] = 1'
End
If (#SearchValue = 'Inactive')
Begin
Set #SearchOption = ' Where a.[Status] = 0'
End
If (#SearchValue = 'All')
Begin
Set #SearchOption = ' Where 1=1'
End
Set #SQLQuery = #SQLQuery + #SearchOption
Exec(#SQLQuery);
Saw this in production code and asked seniors for help.
Their answer:
-We use 1=1 so when we have to add a new condition we can just type
and <condition>
and get on with it.
I do this usually when I am building dynamic SQL for a report which has many dropdown values a user can select. Since the user may or may not select the values from each dropdown, we end up getting a hard time figuring out which condition was the first where clause. So we pad up the query with a where 1=1 in the end and add all where clauses after that.
Something like
select column1, column2 from my table where 1=1 {name} {age};
Then we would build the where clause like this and pass it as a parameter value
string name_whereClause= ddlName.SelectedIndex > 0 ? "AND name ='"+ ddlName.SelectedValue+ "'" : "";
As the where clause selection are unknown to us at runtime, so this helps us a great deal in finding whether to include an 'AND' or 'WHERE'.
Making "where 1=1" the standard for all your queries also makes it trivially easy to validate the sql by replacing it with where 1 = 0, handy when you have batches of commands/files.
Also makes it trivially easy to find the end of the end of the from/join section of any query. Even queries with sub-queries if properly indented.
I first came across this back with ADO and classic asp, the answer i got was: performance.
if you do a straight
Select * from tablename
and pass that in as an sql command/text you will get a noticeable performance increase with the
Where 1=1
added, it was a visible difference. something to do with table headers being returned as soon as the first condition is met, or some other craziness, anyway, it did speed things up.
Using a predicate like 1=1 is a normal hint sometimes used to force the access plan to use or not use an index scan. The reason why this is used is when you are using a multi-nested joined query with many predicates in the where clause where sometimes even using all of the indexes causes the access plan to read each table - a full table scan. This is just 1 of many hints used by DBAs to trick a dbms into using a more efficient path. Just don't throw one in; you need a dba to analyze the query since it doesn't always work.
Here is a use case... however I am not too concerned with the technicalities of why I should or not use 1 = 1.
I am writing a function, using pyodbc to retrieve some data from SQL Server. I was looking for a way to force a filler after the where keyword in my code. This was a great suggestion indeed:
if _where == '': _where = '1=1'
...
...
...
cur.execute(f'select {predicate} from {table_name} where {_where}')
The reason is because I could not implement the keyword 'where' together inside the _where clause variable. So, I think using any dummy condition that evaluates to true would do as a filler.
I have the following query in SQL Server 2005 which works fine:
DECLARE #venuename NVARCHAR(100)
DECLARE #town NVARCHAR(100)
SET #venuename = NULL -- normally these are parameters in the stored proc.
SET #town = 'London'
SELECT COUNT(*) FROM dbo.Venue
WHERE
(#VenueName IS NULL OR CONTAINS((Venue.VenueName), #VenueName))
AND
(#Town IS NULL OR Town LIKE #Town + '%')
It uses short-circuiting when null values are passed for the parameters (there are many more in the real SP than shown in my example).
However after upgrading to SQL 2012, running this query with NULL passed for #VenueName fails with the error "Null or empty full-text predicate" as SQL Server seems to be running (or evaluating) the CONTAINS statement for #VenueName even when #VenueName is set to NULL.
Is there a way to use short-circuiting in 2012 or is this no longer possible? I'd hate to have to rewrite all of my SPs as we've used this technique in dozens of stored procedures across multiple projects over the years.
I do not know much about sql 2012 but can you please try following
DECLARE #venuename NVARCHAR(100)
DECLARE #town NVARCHAR(100)
SET #venuename = '""' -- -- **Yes '""' instead of null**.
SET #town = 'London'
SELECT COUNT(*) FROM dbo.Venue
WHERE
(#VenueName ='""' OR CONTAINS((Venue.VenueName), #VenueName))
AND
(#Town IS NULL OR Town LIKE #Town + '%')
Check out this thread: OR Operator Short-circuit in SQL Server Within SQL server, there is no guarantee that an OR clause breaks early. It's always been that way, so I guess you've just been lucky that it worked with SQL Server 2005.
To workaround your problem, consider using the ISNULL function every time you supply a parameter value that might be NULL, to the CONTAINS function.
This is Perfect Answer.
Let's examine these two statements:
IF (CONDITION 1) OR (CONDITION 2)
..
IF (CONDITION 3) AND (CONDITION 4)
...
If CONDITION 1 is TRUE, will CONDITION 2 be checked?
If CONDITION 3 is FALSE, will CONDITION 4 be checked?
What about conditions on WHERE: does the SQL Server engine optimize all conditions in a WHERE clause? Should programmers place conditions in the right order to be sure that the SQL Server optimizer resolves it in the right manner?
ADDED:
Thank to Jack for link, surprise from t-sql code:
IF 1/0 = 1 OR 1 = 1
SELECT 'True' AS result
ELSE
SELECT 'False' AS result
IF 1/0 = 1 AND 1 = 0
SELECT 'True' AS result
ELSE
SELECT 'False' AS result
There is not raise a Divide by zero exception in this case.
CONCLUSION:
If C++/C#/VB has short-circuiting why can't SQL Server have it?
To truly answer this let's take a look at how both work with conditions. C++/C#/VB all have short circuiting defined in the language specifications to speed up code execution. Why bother evaluating N OR conditions when the first one is already true or M AND conditions when the first one is already false.
We as developers have to be aware that SQL Server works differently. It is a cost based system. To get the optimal execution plan for our query the query processor has to evaluate every where condition and assign it a cost. These costs are then evaluated as a whole to form a threshold that must be lower than the defined threshold SQL Server has for a good plan. If the cost is lower than the defined threshold the plan is used, if not the whole process is repeated again with a different mix of condition costs. Cost here is either a scan or a seek or a merge join or a hash join etc... Because of this the short-circuiting as is available in C++/C#/VB simply isn't possible. You might think that forcing use of index on a column counts as short circuiting but it doesn't. It only forces the use of that index and with that shortens the list of possible execution plans. The system is still cost based.
As a developer you must be aware that SQL Server does not do short-circuiting like it is done in other programming languages and there's nothing you can do to force it to.
I am converting a stored procedure which I had previously written as a string then, using BIT parameters I decided whether to append certain WHERE/ON clauses
This sp is passed a number of comma-separated strings and then some of the dynamic WHERE clauses are like:
IF #pUse_Clause_A THEN SET #WhereClause = #WhereClause + ' AND [FIELD_A] IN (' + #pComma_Separated_List_A + ')'
In this case, #pComma_Separated_List_A is something like '1,3,6,66,22' ... a list of the things I want included.
Now I am changing these from strings into TVP,s so I can just use "real" SQL like
AND [FIELD_A] IN (SELECT [TVP_FIELD] FROM #pTVP_A)
When I do this, I don't like the string-building method
However, I also don't like having to nest the IF statements.
IF A
ENTIRE SQL WHERE A
ELSE
ENTIRE SQL WITHOUT WHERE CLAUSE
The more parameters I add, the more complicated it gets:
IF A
IF B
SQL WHERE A AND B
ELSE
SQL WHERE A
ELSE
IF B
SQL WHERE B
ELSE
SQL
What I would rather do is something like this:
SELECT * FROM TABLE
WHERE 1=1
CASE USE_A WHEN 1 THEN
AND [FIELD_A] IN (SELECT A FROM TBP_A)
END
CASE USE_B WHEN 1 THEN
AND [FIELD_B] IN (SELECT B FROM TVP_B)
END
I know it ignored SQL outside the chosen "IF" result, but having all that duplicated statement seems sloppy
Dynamically changing searches based on the given parameters is a complicated subject and doing it one way over another, even with only a very slight difference, can have massive performance implications. The key is to use an index, ignore compact code, ignore worrying about repeating code, you must make a good query execution plan (use an index).
Read this and consider all the methods. Your best method will depend on your parameters, your data, your schema, and your actual usage:
Dynamic Search Conditions in T-SQL by by Erland Sommarskog
The Curse and Blessings of Dynamic SQL by Erland Sommarskog
I have a table with 10 columns as col_1,col_2,.... col_10. I want to write a select statement that will select a value of one of the row and from one of these 10 columns. I have a variable that will decide which column to select from. Can such query be written where the column name is dynamically decided from a variable.
Yes, using a CASE statement:
SELECT CASE #MyVariable
WHEN 1 THEN [Col_1]
WHEN 2 THEN [Col_2]
...
WHEN 10 THEN [Col_10]
END
Whether this is a good idea is another question entirely. You should use better names than Col_1, Col_2, etc.
You could also use a string substitution method, as suggested by others. However, that is an option of last resort because it can open up your code to sql injection attacks.
Sounds like a bad, denormalized design to me.
I think a better one would have the table as parent, with rows that contain a foreign key to a separate child table that contains ten rows, one for each of those columns you have now. Let the parent table set the foreign key according to that magic value when the row is inserted or updated in the parent table.
If the child table is fairly static, this will work.
Since I don't have enough details, I can't give code. Instead, I'll explain.
Declare a string variable, something like:
declare #sql varchar(5000)
Set that variable to be the completed SQL string you want (as a string, and not actually querying... so you embed the row-name you want using string concatenation).
Then call: exec(#sql)
All set.
I assume you are running purely within Transact-SQL. What you'll need to do is dynamically create the SQL statement with your variable as the column name and use the EXECUTE command to run it. For example:
EXECUTE('select ' + #myColumn + ' from MyTable')
You can do it with a T-SQl CASE statement:
SELECT 'The result' =
CASE
WHEN choice = 1 THEN col1
WHEN choice = 2 THEN col2
...
END
FROM sometable
IMHO, Joel Coehoorn's case statement is probably the best idea
... but if you really have to use dynamic SQL, you can do it with sp_executeSQL()
I have no idea what platform you are using but you can use Dynamic LINQ pretty easily to do this.
var query = context.Table
.Where( t => t.Id == row_id )
.Select( "Col_" + column_id );
IEnumerator enumerator = query.GetEnumerator();
enumerator.MoveNext();
object columnValue = enumerator.Current;
Presumably, you'll know which actual type to cast this to depending on the column. The nice thing about this is you get the parameterized query for free, protecting you against SQL injection attacks.
This isn't something you should ever need to do if your database is correctly designed. I'd revisit the design of that element of the schema to remove the need to do this.