MYSQL if a select query returns 0 rows then another select query?

Question

if select * from table where x=1 returns 0 rows, then I need select * from table where x=2 [or some other query]. Is it possible to do this in a si

Answer 1

SQL_CALC_FOUND_ROWS and FOUND_ROWS cannot be used in a single query, even if separate by UNION statements.

The correct way to do this would be:

WITH  my_cte AS
(
  SELECT * from original_set
)
SELECT * FROM my_cte
UNION ALL
SELECT opt.* FROM optional_set opt JOIN (SELECT count(*) v FROM my_cte) count ON count.v=0;

With the JOIN and the UNION ALL the performance of this query is almost equivalent to either of the individual standalone queries

Answer 2

You could try...

SELECT *
    FROM mytable
    WHERE x = 1

UNION

SELECT *
    FROM mytable
    WHERE x = 2 AND
          NOT EXISTS (SELECT *
                          FROM mytable
                          WHERE x = 1);

if you don't consider it too ghastly a hack.

Answer 3

yes

Subqueries with EXISTS or NOT EXISTS

http://dev.mysql.com/doc/refman/5.1/en/exists-and-not-exists-subqueries.html

example :

SELECT column1 FROM t1 WHERE NOT EXISTS (SELECT * FROM t2);

Answer 4

If the two queries return different number of columns, you can pad one of the results with empty columns and also add an identifier column first.

SELECT SQL_CALC_FOUND_ROWS 1 query_type, mytable.*, 
'' col1, '' col2, '' col3, '' col4
FROM mytable
WHERE x = 1

UNION ALL

SELECT 2, mytable2.*
FROM mytable2
WHERE 
FOUND_ROWS() = 0 AND x = 2;

Where mytable2 has 4 more columns than mytable.

Answer 5

This appears to work from a quick test I just did and avoids the need to check for the existence of x=1 twice.

SELECT SQL_CALC_FOUND_ROWS *
FROM mytable
WHERE x = 1

UNION ALL

SELECT *
FROM mytable
WHERE 
FOUND_ROWS() = 0 AND x = 2;

Edit: Following your clarification to the question obviously the 2 queries will need to be UNION compatible for the above to work.

The answer to your updated question is No. This is not possible in a single query. You would need to use some conditional procedural logic to execute the desired query.

Answer 6

The simplest explanation is that:

SELECT IF(1 = 2,'true','false'); --> false
SELECT IF(1 = 1,' true','false'); --> true
SELECT IF(1 = 2,' true','false'), IF(1 = 1,' true','false'); --> false | true

The 'if' statement give some functionality to selected values. The structure is something like this:

SELECT IF(<your statement>), ...<selected params>... FROM <your tables>

A great explanation can be found here.