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How to select an empty result set?

Posted by: admin November 7, 2017 Leave a comment

Questions:

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 more elegant 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.

Answers:

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.

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This should work for any DB, tested on Postgres and Netezza:

SELECT NULL LIMIT 0;

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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.

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T-SQL (MSSQL):

SELECT Top 0 1;

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This will probably work across all databases.

SELECT * FROM (SELECT NULL AS col0) AS inner0 WHERE col0 IS NOT NULL;

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SELECT TOP 0 * FROM [dbo].[TableName]

This is a reasonable approach to constant scan operator.

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How about this?

SELECT 'MyName' AS EmptyColumn
FROM dual
WHERE 'Me' = 'Funny'

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In PostgreSQL a simple

SELECT;

works. You won’t even get any columns labeled ‘unknown’.
Note however, it still says 1 row retrieved.

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Answers:
SELECT * FROM (SELECT NULL) WHERE 0