在mysql中设置结果的情况

Question

I want's to test following query in mysql

SELECT result.* FROM 
(CASE WHEN (2 = 2) 
THEN
(SELECT * FROM mytable WHERE myID =2814 )
END) result ;

but it is sending syntax error. Any idea what's wrong with it ?

Answer 1

you can do it in a round about way like so

SET @query := NULL;

SELECT @query := t.qry 
FROM 
(   SELECT 
        CASE WHEN 2 = 2
             THEN "SELECT * FROM mytable WHERE myID =2814"
             WHEN another_condition
             THEN "another_select"
        END as qry
) t ;

PREPARE query1 FROM @query;
EXECUTE query1;

Answer 2

This is too long for a comment.

Obviously, your query is equivalent to:

SELECT *
FROM mytable
WHERE myID = 2814;

This makes it a bit hard to figure out what you are asking.

Fundamentally, though case is an expression, and the from clause does not accept expressions. It accepts table names, view names, subqueries, and the like. Second, the case statement has a few other problems.

• It returns a scalar scalar value. The subquery could return more than one row.
• Returning more than one column is not allowed
• All the clauses need to return the same thing

If you wanted to select from multiple tables -- and all of them have the same structure -- you can use a union all construct:

select t.*
from table2 t
where . . .
union all
select t.*
from table2 t
where . . .

Answer 3

I would prefer to use a different query each time (which at least will allow mysql some chance of using indexes properly).

However assuming there are several sub queries that you want as a source for data depending on the check then maybe something like this:-

SELECT result.* 
FROM 
(
    SELECT * FROM mytable WHERE 2 = 2 AND myID = 2814  
    UNION
    SELECT * FROM mytable2 WHERE 1 = 2 AND myID = 2814  
    UNION
    SELECT * FROM mytable3 WHERE 3 = 2 AND myID = 2814  
) result ;