I was wondering if something like this was possible:
SELECT name or alt_name FROM users Where userid=somenum
I have also tried this:
SELECT name IF NOT NULL ELSE alt_name ...
If anyone has any insight I'd appreciate it.
If your trying to replace nulls then
select coalesce(name,alt_name)....
it will return first non null value
How about this?
SELECT IsNull(name, alt_name) FROM users Where userid=somenum
Its similar to the Coalesce function, but it only takes two arguments and is easier to spell.
Do you mean something like this? I'm assuming TSQL
SELECT CASE WHEN [name] IS NOT NULL THEN [name] ELSE [alt_name] END AS [UserName]
FROM [Users] WHERE [UserId] = somenum
Not the most efficient answer, but what about this:
SELECT name FROM users WHERE userid=somenum AND name IS NOT NULL
UNION
SELECT altname FROM users WHERE userid=somenum AND name IS NULL
I think this will produce what you are after.
SELECT CASE
WHEN userid = somenum THEN name
ELSE alt_name
END
FROM users
If you're using a database that supports them then one of the simplest ways is to use a user defined function, you'd then have
SELECT udfPreferredName() FROM users
where udfPreferredName() would encapsulate the logic required to choose between the name and alternative_name fields.
One of the advantages of using a function is that you can abstract away the choice logic and apply it in multiple SQL statements wherever you need it. Doing the logic inline using a case is fine, but will usually be (much) harder to maintain across a system. In most RDBMS the additional overhead of the function call will not be significant unless you are handling very large tables
Related
To start, take this snippet as an example:
SELECT *
FROM StatsVehicle
WHERE ((ReferenceMakeId = #referenceMakeId)
OR #referenceMakeId IS NULL)
This will fetch and filter the records if the variable #referenceMakeId is not null, and if it is null, will fetch all the records. In other words, it is taking the first one into consideration if #referenceMakeId is not null.
I would like to add a further restriction to this, how can I achieve this?
For instance
(ReferenceModelId = #referenceModeleId) OR
(
(ReferenceMakeId = #referenceMakeId) OR
(#referenceMakeId IS NULL)
)
If #referenceModelId is not null, it will only need to filter by ReferenceModelId, and ignore the other statements inside it. If I actually do this as such, it returns all the records. Is there anything that can be done to achieve such a thing?
Maybe something like this?
SELECT * FROM StatsVehicle WHERE
(
-- Removed the following, as it's not clear if this is beneficial
-- (#referenceModeleId IS NOT NULL) AND
(ReferenceModelId = #referenceModeleId)
) OR
(#referenceModeleId IS NULL AND
(
(ReferenceMakeId = #referenceMakeId) OR
(#referenceMakeId IS NULL)
)
)
This should do the trick.
SELECT * FROM StatsVehicle
WHERE ReferenceModelId = #referenceModeleId OR
(
#referenceModeleId IS NULL AND
(
#referenceMakeId IS NULL OR
ReferenceMakeId = #referenceMakeId
)
)
However, you should note that this types of queries (known as catch-all queries) tend to be less efficient then writing a single query for every case.
This is due to the fact that SQL Server will cache the first query plan that might not be optimal for other parameters.
You might want to consider using the OPTION (RECOMPILE) query hint, or braking down the stored procedure to pieces that will each handle the specific conditions (i.e one select for null variables, one select for non-null).
For more information, read this article.
If #referenceModelId is not null, it will only need to filter by
ReferenceModelId, and ignore the other statements inside it. If I
actually do this as such, it returns all the records. Is there
anything that can be done to achieve such a thing?
You can think of using a CASE for good short circuit mechanism
WHERE
CASE
WHEN #referenceModelId is not null AND ReferenceModelId = #referenceModeleId THEN 1
WHEN #referenceMakeId is not null AND ReferenceMakeId = #referenceMakeId THEN 1
WHEN #referenceModelId is null AND #referenceMakeId is null THEN 1
ELSE 0
END = 1
I am in a situation where I need to return results if some conditions on the string/character are met.
For example: to return only the names that contain 'F' character from the Person table.
How to create an SQL query based on such conditions? Is there any link to a documentation that explains how can SQL perform such queries?
Thanks in advance
The most basic approach is to use LIKE operator:
-- name starts with 'F'
SELECT * FROM person WHERE name LIKE 'F%'
-- name contains 'F'
SELECT * FROM person WHERE name LIKE '%F%'
(% is a wildcard)
Most RDBMS offer string operations which are able to perform that required task in one way or the other.
In MySQL you might use INSTR:
SELECT *
FROM yourtable
WHERE INSTR(Person, 'F') > 0;
In Oracle, this can be done, too.
In PostgreSQL, you can use STRPOS:
SELECT *
FROM yourtable
WHERE strpos(Person, 'F') > 0;
Usually there are several approaches to solve this, many would choose the LIKE operator. For more details, please refer to the documentation of the RDBMS of your choice.
Update
As requested by the questioner a few words about the LIKE operator, which are used not only in MySQL or Oracle, but in other RDBMS, too.
The use of LIKE will in some cases make your RDBMS try to use an index, it usually does not not try to do so if you use a string functions.
Example:
SELECT *
FROM yourtable
WHERE Person LIKE 'F%';
The query may look like this:
SELECT * FROM Person WHERE FirstName LIKE '%F%' OR LastName LIKE '%F%'
I use this pattern for optional filter paramaters in my SQL Stored Procedures
AND (
#OptionalParam IS NULL OR
(
Id = #OptionalParam
)
)
However the OR is not a friend of the query optimizer. Is there a more efficient way to do this without using dynamic SQL
You can try using COALESCE. Not sure if it will be more efficient.
AND Id = Coalesce(#OptionalParam, Id)
This will not work if Id itself is null and you are using ANSI nulls.
AND ID = ISNULL(#OptionalParam, ID)
or you if you had multiple optional parameters can use
AND ID = COALESCE(#OptionalParam1, #OptionalParam2, ID)
This is definitely faster than using an OR statement.
Like the other answerer mentioned, this will not work if the ID column is null (but then again, the original statement wouldn't either).
You could try:
AND Id = CASE WHEN #OptionalParam IS NULL THEN Id ELSE NULL END
I doubt this will optimize much better, but there's no OR in it.
Alternatively, you could break your query apart into two components -- one with just an #OptionalParam IS NULL test and another with an Id = #OptionalParam test, then UNION them together. Depending on your data topology this might yield better results. It could also be significantly worse.
Writing a stored procedure that will have multiple input parameters. The parameters may not always have values and could be empty. But since the possibility exists that each parameter may contain values I have to include the criterion that utilizing those parameters in the query.
My query looks something like this:
SELECT DISTINCT COUNT(*) AS SRM
FROM table p
WHERE p.gender IN (SELECT * FROM Fn_SplitParms(#gender)) AND
p.ethnicity IN (SELECT * FROM Fn_SplitParms(#race)) AND
p.marital_status IN (SELECT * FROM Fn_SplitParms(#maritalstatus))
So my problem is if #gender is empty(' ') the query will return data where gender field is empty when I really want to just ignore p.gender all together. I don't want to have to accomplish this task using IF/ELSE conditional statements because they would be too numerous.
Is there any way to use CASE with IN for this scenario? OR
Is there other logic that I'm just not comprehending that will solve this?
Having trouble finding something that works well...
Thanks!
Use or:
SELECT DISTINCT COUNT(*) AS SRM
FROM table p
WHERE
(p.gender IN (SELECT * FROM Fn_SplitParms(#gender)) OR #gender = '')
AND (p.ethnicity IN (SELECT * FROM Fn_SplitParms(#race)) OR #race = '')
AND (p.marital_status IN (SELECT * FROM Fn_SplitParms(#maritalstatus)) OR #maritalstatus = '')
You might also want to consider table-valued parameters (if using SQL Server 2008 and up) - these can sometimes make the code simpler, since they are treated as tables (which in your case, may be empty) and you can join - plus no awkward split function required.
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I'm using a stored procedure in MySQL, with a CASE statement.
In the ELSE clause of the CASE ( equivalent to default: ) I want to select and return an empty result set, thus avoiding to throw an SQL error by not handling the ELSE case, and instead return an empty result set as if a regular query would have returned no rows.
So far I've managed to do so using something like:
Select NULL From users Where False
But I have to name an existing table, like 'users' in this example.
It works, but I would prefer a way that doesn't break if eventually the table name used is renamed or dropped.
I've tried Select NULL Where False but it doesn't work.
Using Select NULL does not return an empty set, but one row with a column named NULL and with a NULL value.
There's a dummy-table in MySQL called 'dual', which you should be able to use.
select
1
from
dual
where
false
This will always give you an empty result.
This should work on most DBs, tested on Postgres and Netezza:
SELECT NULL LIMIT 0;
T-SQL (MSSQL):
SELECT Top 0 1;
How about
SELECT * FROM (SELECT 1) AS TBL WHERE 2=3
Checked in myphp, and it also works in sqlite and probably in any other db engine.
This will probably work across all databases.
SELECT * FROM (SELECT NULL AS col0) AS inner0 WHERE col0 IS NOT NULL;
SELECT TOP 0 * FROM [dbo].[TableName]
This is a reasonable approach to constant scan operator.
SELECT NULL WHERE FALSE;
it works in postgresql ,mysql, subquery in mysql.
How about this?
SELECT 'MyName' AS EmptyColumn
FROM dual
WHERE 'Me' = 'Funny'
SELECT * FROM (SELECT NULL) WHERE 0
In PostgreSQL a simple
SELECT;
works. You won't even get any columns labeled 'unknown'.
Note however, it still says 1 row retrieved.