How to set -- Comments inside the variable declare? - sql

Question

SELECT * FROM CUSTOMERS
--WHERE ID='1'
I want the result like above statement, anyone can advice this can be declare comments or any other way?
DECLARE #TEST as VARCHAR(2) ='--'
SELECT * FROM CUSTOMERS
#TEST WHERE ID='1'

Answer 1

There is no preprocessor that can do what you want, and if there would be a preprocessor, it would probably only work in specific client software (like SQL Server Management Studio).
The best I can come up with is declaring a BIT variable and include it in your WHERE statement's logic.
DECLARE #TEST BIT = 0; -- Change to 1 for testing (which will show all records instead of only the record with ID = '1')
SELECT * FROM CUSTOMERS
WHERE ID = '1' OR #TEST = 1;
But using ORs in a WHERE-clause can become a performance issue (due to not using indexes), so using a UNION might be a solution in that case:
DECLARE #TEST BIT = 0; -- Change to 1 for testing (which will show all records instead of only the record with ID = '1')
SELECT * FROM CUSTOMERS
WHERE ID = '1'
UNION
SELECT * FROM CUSTOMERS
WHERE #TEST = 1;
I would personally go for the first option and fall back to the second option if that would produce a better query execution plan.
The latter solution does have the drawback that you have to duplicate your query logic, so changes have to be made in two places instead of one.
It's also the question if you would like to include such a testing variable at all. You can include a complete test query in comments:
/*
-- For testing purposes:
SELECT * FROM CUSTOMERS
*/
SELECT * FROM CUSTOMERS
WHERE ID = '1';
In SSMS, you can simply select the text inside the comment block and hit F5. Only the selected statement(s) will be parsed and executed.
Finally, if you have a large script with many testing cases, you could consider using a separate test script for that. In case of changes, you would have to maintain two scripts, of course, but everything related to testing is cleanly separated that way.

Answer 2

Try following statement:
DECLARE #TEST BIT = 1
DECLARE #ID NVARCHAR(50) = N'1'
SELECT *
CUSTOMERS as c
WHERE #TEST = 1 OR c.ID = #ID
So you can test without commenting out...
Another option is dynamic statements:
DECLARE #Statement NVARCHAR(4000) = NULL
DECLARE #TEST BIT = 1
IF #TEST = 1
BEGIN
SET #Statement = N'SELECT * FROM CUSTOMERS AS c'
END
ELSE
BEGIN
SET #Statement = N'SELECT * FROM CUSTOMERS AS c WHERE c.ID = #ID'
END
EXEC sp_executesql #Statement,
N'#ID NVARCHAR(50)',
#ID = N'1'

Answer 3

By Using Dynamic SQL Query.You can Set your Common SQL Command in One Variable and the based on your need/Requirement you can CONCAT the String and this can be achieved from by using Dynamic SQL
DECLARE #TEST VARCHAR(MAX),#Where VARCHAR(MAX)=' WHERE ID=''1'''
IF(Condition Check)
SET #TEST='SELECT * FROM CUSTOMERS' +#Where
ELSE
SET #TEST='SELECT * FROM CUSTOMERS'
SELECT #TEST
EXEC (#TEST)
Another Option:
DECLARE #TEST VARCHAR(2) =NULL
SET #TEST='1'
SELECT * FROM CUSTOMERS
WHERE (ID=#TEST OR #TEST IS NULL)

Answer 4

For quick debugging I'm using pattern like next:
declare #var int -- can be some function or procedure param
select f1, f2, ... -- can be update or delete too
--declare #var int = 7; select *
from mytable
where id = #var