I have a table, which has columns, say
Week1,Week2, Week3 and so on.
I have a stored procedure, and based on the number input, i want to select that column.
Example, if input is 4 then I want to make the query,
select *
from table_name
where Week4=<something>
Is there any way to do this other than using dynamic query? Because this dynamic thing will be just a small part of a huge query.
The comments about normalization are right, but if you have no choice, you can use "or" clauses:
declare #inputvalue int;
set #int = 1;
select *
from <table>
where (week1 = <something> and #inputvalue = 1)
or (week2 = <something> and #inputvalue = 2)
or (week3 = <something> and #inputvalue = 3)
or (week4 = <something> and #inputvalue = 4)
This will be very slow if the tables are of any size, as you won't be using any indexes. I wouldn't suggest doing this unless you're absolutely unable to change the table structure.
I realize this isn't what you asked for, but I figured I'd point out to some people who find this what you mean by doing this as a dynamic query.
You'd just write a procedure and hold the field name in there. Assuming that the naming standard is the same, so the input value would be the week# (1,2,7,27, 123, etc.) and the field name would directly correspond (Week1, Week2, Week7, Week27, Week123, etc.)
create or replace procedure myweek(week_in varchar2)
is
dyn_sql varchar2(1000);
begin
dyn_sql := 'select * from table_name where week'||week_in||' = ''something;'' '
execute immediate dyn_sql;
end;
/
Then to call it you'd just do something like :
exec myweek(27); and it would generate the sql:
select * from table_name where week27 = 'something';
Related
I'm creating an update statement that generate SHA256 for table columns based on table's name
1st Step: I created a procedure that get the table columns, concatenate it all in one columns, then format to a desired format.
-- Procedure code : Extract table's columns list, concatenate it and format it
Create procedure SHA_PREP (in inp1 nvarchar(20))
as
begin
SELECT concat(concat('hash_sha256(',STRING_AGG(A, ', ')),')') AS Names
FROM (
SELECT concat('to_varbinary(IFNULL("',concat(COLUMN_NAME,'",''0''))')) as A
FROM SYS.TABLE_COLUMNS
WHERE SCHEMA_NAME = 'SCHEMA_NAME' AND TABLE_NAME = :inp1
AND COLUMN_NAME not in ('SHA')
ORDER BY POSITION
);
end;
/* Result of this procedures :
hash_sha256(
to_varbinary("ID"),to_varbinary(IFNULL("COL1",'0')),to_varbinary(IFNULL("COL2",'0')) )
*/
-- Update Statement needed
UPDATE "SCHEMA_NAME"."TABLE_NAME"
SET "SHA" = CALL "SCHEMA_NAME"."SHA_PREP"('SCHEMA_NAME')
WHERE "ID" = 99 -- a random filter
The solution by #SonOfHarpy technically works but has several issues, namely:
unnecessary use of temporary tables
overly complicated string assignment approach
use of fixed system table schema (SYS.TABLE_COLUMNS) instead of PUBLIC synonym
wrong data type and variable name for the input parameter
An improved version of the code looks like this:
create procedure SHA_PREP (in TABLE_NAME nvarchar(256))
as
begin
declare SQL_STR nvarchar(5000);
SELECT
'UPDATE "SCHEMA_NAME"."TABLE_NAME" SET "SHA"= hash_sha256(' || STRING_AGG(A, ', ') || ')'
into SQL_STR
FROM (
SELECT
'TO_VARBINARY(IFNULL("'|| "COLUMN_NAME" ||'",''0''))' as A
FROM TABLE_COLUMNS
WHERE
"SCHEMA_NAME" = 'SCHEMA_NAME'
AND "TABLE_NAME" = :TABLE_NAME
AND "COLUMN_NAME" != 'SHA'
ORDER BY POSITION
);
-- select :sql_str from dummy; -- this is for debugging output only
EXECUTE IMMEDIATE (:SQL_STR);
end;
By changing the CONCAT functions to the shorter || (double-pipe) operator, the code becomes a lot easier to read as the formerly nested function calls are now simple chained concatenations.
By using SELECT ... INTO variable the whole nonsense with the temporary table can be avoided, again, making the code easier to understand and less prone to problems.
The input parameter name now correctly reflects its meaning and mirrors the HANA dictionary data type for TABLE_NAME (NVARCHAR(256)).
The procedure now consists of two commands (SELECT and EXECUTE IMMEDIATE) that each performs an essential task of the procedure:
Building a valid SQL update command string.
Executing the SQL command.
I removed the useless line-comments but left a debugging statement as a comment in the code, so that the SQL string can be reviewed without having to execute the command.
For that to work, obviously, the EXECUTE... line needs to be commented out and the debugging line has to be uncommented.
What's more worrying than the construction of the solution is its purpose.
It looks as if the SHA column should be used as a kind of shorthand row-data fingerprint. The UPDATE approach certainly handles this as an after-thought activity but leaves the "finger-printing" for the time when the update gets executed.
Also, it takes an essential part of the table design (that the SHA column should contain the fingerprint) away from the table definition.
An alternative to this could be a GENERATED COLUMN:
create table test (aaa int, bbb int);
alter table test add (sha varbinary (256) generated always as
hash_sha256(to_varbinary(IFNULL("AAA",'0'))
, to_varbinary(IFNULL("BBB",'0'))
)
);
insert into test (aaa, bbb) values (12, 32);
select * from test;
/*
AAA BBB SHA
12 32 B6602F58690CA41488E97CD28153671356747C951C55541B6C8D8B8493EB7143
*/
With this, the "generator" approach could be used for table definition/modification time, but all the actual data handling would be automatically done by HANA, whenever values get changed in the table.
Also, no separate calls to the procedure will ever be necessary as the fingerprints will always be current.
I find a solution that suits my need, but maybe there's other easier or more suitable approchaes :
I added the update statement to my procedure, and inserted all the generated query into a temporary table column, the excuted it using EXECUTE IMMEDIATE
Create procedure SHA_PREP (in inp1 nvarchar(20))
as
begin
/* ********************************************************** */
DECLARE SQL_STR VARCHAR(5000);
-- Create a temporary table to store a query in
create local temporary table #temp1 (QUERY varchar(5000));
-- Insert the desirable query into the QUERY column (Temp Table)
insert into #temp1(QUERY)
SELECT concat('UPDATE "SCHEMA_NAME"."TABLE_NAME" SET "SHA" =' ,concat(concat('hash_sha256(',STRING_AGG(A, ', ')),')'))
FROM (
SELECT concat('to_varbinary(IFNULL("',concat(COLUMN_NAME,'",''0''))')) as A
FROM SYS.TABLE_COLUMNS
WHERE SCHEMA_NAME = 'SCHEMA_NAME' AND TABLE_NAME = :inp1
AND COLUMN_NAME not in ('SHA')
ORDER BY POSITION
);
end;
/* QUERY : UPDATE "SCHEMA_NAME"."TABLE_NAME" SET "SHA" =
hash_sha256(to_varbinary("ID"),to_varbinary(IFNULL("COL1",'0')),to_varbinary(IFNULL("COL2",'0'))) */
SELECT QUERY into SQL_STR FROM "SCHEMA_NAME".#temp1;
--Excuting the query
EXECUTE IMMEDIATE (:SQL_STR);
-- Dropping the temporary table
DROP TABLE "SCHEMA_NAME".#temp1;
/* ********************************************************** */
end;
Any other solution or improvement are well welcomed
Thank you
Is it possible to test for a column before selecting it within a select statement?
This may be rough for me to explain, I have actually had to teach myself dynamic SQL over the past 4 months. I am using a dynamically generated parameter (#TableName) to store individual tables within a loop (apologize for the vagueness, but the details aren't relevant).
I then want to be able to be able to conditionally select a column from the table (I will not know if each table has certain columns). I have figured out how to check for a column outside of a select statement...
SET #SQLQuery2 = 'Select #OPFolderIDColumnCheck = Column_Name From INFORMATION_SCHEMA.COLUMNS Where Table_Name = #TABLENAME And Column_Name = ''OP__FolderID'''
SET #ParameterDefinition2 = N'#TABLENAME VARCHAR(100), #OPFolderIDColumnCheck VARCHAR(100) OUTPUT'
EXECUTE SP_EXECUTESQL #SQLQuery2, #ParameterDefinition2, #TABLENAME, #OPFolderIDColumnCheck OUTPUT
IF #OPFolderIDColumnCheck IS NULL
BEGIN
SET #OP__FOLDERID = NULL
END
ELSE
IF #OPFolderIDColumnCheck IS NOT NULL
BEGIN
...etc
but id like to be able to do it inside of a select statement. Is there a way to check and see if OP__FOLDERID exists in the table?
Id like to be able to do something like this:
SELECT IF 'OP__FOLDERID' EXISTS IN [TABLE] THEN 'OP__FOLDERID' FROM [TABLE]
Thank you for any help or direction you can offer.
I'm afraid there isn't any direct way to do this within a SELECT statement at all. You can determine if a column exists in a table, however, and construct your dynamic SQL accordingly. To do this, use something like this:
IF COL_LENGTH('schemaName.tableName', 'columnName') IS NOT NULL
BEGIN
-- Column Exists
END
You could then set a variable as a flag, and the code to construct the dynamic SQL would construct the expression with/without the column, as desired. Another approach would be to use a string value, and set it to the column name if it is present (perhaps with a prefix or suffix comma, as appropriate to the expression). This would allow you to save writing conditionals in the expression building, and would be particularly helpful where you have more than one or two of these maybe-columns in a dynamic expression.
i want to use the result of a query as an input in another query.
What might make it difficult: The variable is the schema in the database.
CREATE or replace VARIABLE myschema varchar(15) ;
set myschema = (select owner from syscat.tables where tabname = 'xyz');
select count(name) as result from myschema.USR02 where USTYP = 'A';
DROP VARIABLE myschema;
This is my last try, after i failed using declare.
But i get an error, because "myschema" is used as a string, and of course there is no schema with name "myschema". The result of the first query is not used.
If I just run the first two lines, i get the schemaname as result. Do i have to mark the variable in a special way? The goal is just the result of the query in line 3 by using the dynamic value of "myschema".
Unfortunately, you have to use dynamic SQL (forming a custom SQL query through string manipulation) if you want to deal with table, schema, or column names dynamically:
This is the basic idea:
execute immediate 'select * from ' || myschema || '.USR02';
However, you can't just run a bare select in dynamic SQL; you have to put the result in something. And the whole thing must be in a compound SQL block. So the full example would look something like this (simplified query for space).
This query assumes that a table called "result" exists to store the result you are returning.
begin
declare myschema varchar(100) default '';
set myschema = (select owner from syscat.tables where tabname = 'xyz');
execute immediate 'insert into result select count(*) from ' || myschema || '.USR02';
end
select * from result;
Note that within the block, you can simply declare a variable (as shown in my example). So you don't have to declare a global variable for this purpose, unless you want it to persist beyond this one statement.
I'm working on a e-learning project in which there is a table named chapter in which there is a column named question_table this is table in which the specific chapter's questions are added.
Now the problem is I want to display all the question from all the chapter for this I used following sql query
SELECT * FROM (SELECT `question_table` FROM `chapter`)
but it doesn't work and gives the error:
"Every derived table must have its own alias".
Note: I want to do it using SQL not PHP.
Firstly, I think you would be better redesigning your database. Multiple tables of the same structure holding the same data are generally not a good idea.
However what you require is possible using a MySQL procedure to build up some dynamic SQL and then execute it, returning the resulting data.
A procedure as follows could be used to do this:-
DROP PROCEDURE IF EXISTS dynamic;
delimiter //
CREATE PROCEDURE dynamic()
BEGIN
DECLARE question_table_value VARCHAR(25);
DECLARE b INT DEFAULT 0;
DECLARE c TEXT DEFAULT '';
DECLARE cur1 CURSOR FOR SELECT `question_table` FROM `chapter`;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET b = 1;
OPEN cur1;
SET b = 0;
WHILE b = 0 DO
FETCH cur1 INTO question_table_value;
IF b = 0 THEN
IF c = '' THEN
SET c = CONCAT('SELECT * FROM `',question_table_value, '`');
ELSE
SET c = CONCAT(c, ' UNION SELECT * FROM `',question_table_value, '`');
END IF;
END IF;
END WHILE;
CLOSE cur1;
SET #stmt1 := c;
PREPARE stmt FROM #stmt1;
EXECUTE stmt;
END
This is creating a procedure called dynamic. This takes no parameters. It sets up a cursor to read the question_table column values from the chapter table. It looks around the results from that, building up a string which contains the SQL, which is a SELECT from each table with the results UNIONed together. This is then PREPAREd and executed. The procedure will return the result set from the SQL executed by default.
You can call this to return the results using:-
CALL dynamic()
Down side is that this isn't going to give nice results if there are no rows to return and they are not that easy to maintain or debug with the normal tools developers have. Added to which very few people have any real stored procedure skills to maintain it in future.
In MySQL you must give every subquery ("derived table") an alias:
SELECT * FROM (SELECT question_table FROM chapter) t --notice the alias "t"
The derived table here is the result of the (SELECT ...). You need to give it an alias, like so:
SELECT * FROM (SELECT question_table FROM chapter) X;
Edit, re dynamic tables
If you know all the tables in advance, you can union them, i.e.:
SELECT * FROM
(
SELECT Col1, Col2, ...
FROM Chapter1
UNION
SELECT Col1, Col2, ...
FROM Chapter2
UNION
...
) X;
SqlFiddle here
To do this solution generically, you'll need to use dynamic sql to achieve your goal.
In general however, this is indicative of a smell in your table design - your chapter data should really be in one table, and e.g. classified by the chapter id.
If you do need to shard data for scale or performance reasons, the typical mechanism for doing this is to span multiple databases, not tables in the same database. MySql can handle large numbers of rows per table, and performance won't be an issue if the table is indexed appropriately.
In SQL Server 2005 or later you can use following syntax to return a a variable number of rows:
Declare #Limit Int
Set #Limit=5
Select Top(#Limit) * From TableName
Is there some magic value, that you can use to let it return all rows? (Thinking of parametrized stored procedure here). Setting limit as 0 will just return no rows, and using negative value will generate run-time error.
I am pretty sure this is not possible, but I haven't found a definite answer. Having this work without If/Else block and duplicating the rather complicated query would be nice.
The simple answer would be to convert to bigint and use its maximum possible value (9223372036854775807). Since that many rows cannot possibly be part of any table, you can be sure you won't miss anything.
To avoid rewriting the query, which I'm assuming is more complicated than the SQL in the question, you could return the row count to replace the variable in a certain case, for example, if the variable = 0, then return all:
declare #Limit Int
set #Limit = 0
if #Limit = 0
// get the row count from the table you are querying if #Limit = 0
select #Limit = count(1) from TABLE_NAME
// then use the value in your query as before
select top(#Limit) * from TABLE_NAME
A way I would do this is to conditionally use TOP when limit is not -1 so that when you want all results it can be done.
Declare #Limit Int
Set #Limit=5
IF #Limit = -1
SELECT * FROM tbl_Products
ELSE
Select Top(#Limit) * From tbl_Products
Instead of using a number use TOP 100 Percent in your top clause. Let the percentage handle the numbers.
SELECT TOP 100 PERCENT *
FROM TABLE_Name
If you plan to use this inside a stored procedure simply use IF..ELSE Logic as follows .....
CREATE PROCEDURE TestProc
#Top_N INT = NULL
AS
BEGIN
SET NOCOUNT ON;
IF (#Top_N IS NULL)
BEGIN
SELECT * FROM TABLE_NAME
END
ELSE
BEGIN
SELECT TOP (#Top_N) * FROM TABLE_NAME
END
END
Using Percent pass default value of 100% as follows
CREATE PROCEDURE TestProc
#Top_N INT = 100
AS
BEGIN
SET NOCOUNT ON;
SELECT TOP (#Top_N) PERCENT * FROM TABLE_NAME
END