If-then-else 逻辑决定运行哪个选择语句

Question

I'm working on some scripting, and I'm trying to figure out how to add the proper logic so that depending on what a variable that is passed in is (I'll use x as an example), that one of two select statements will run. Here's a simple example of what I'm looking for:

two select statements options X equals either "1" or "2"

if X equals 1 then run "select * from table where y=3 and z=4;"

if X equals 2 then run "select * from table where y=3 and z=5 and q=6 and r=12;"

The fact that the where clauses are different for the two statements is causing me issues since it makes it harder to do a CASE statement how I normally would.

The first thing that came to my mind was to do something that would do something like a case statement in the from clause so that I could do something like:

    select * from 
    (
    case when x=1 then (select 1 from dual)
    case when x=2 then (select 2 from dual)
    )

But I know this doesn't work and I'm not sure of anyway to add this kind of logic to a from clause.

To summarize, I'm trying to run one of two possible queries depending on the value of a variable that is passed into the SQL, but the two queries have different where clauses that make it harder to have a case statement in the where clause. I'd appreciate any input or suggestions on how to tackle this issue.

Thanks in advanced guys!

Answer 1

Use simple OR :

SELECT *
FROM your_table
WHERE (:X = 1 AND y=3 AND z=4)
   OR (:X = 2 AND y=3 and z=5 and q=6 and r=12);

Answer 2

My suggestion is not necessarily the most efficient way to do it especially in your case, because you have a relatively simple task in mind. However, it is worth mentioning that for this case and even more complicated cases you can always write a procedure:

drop procedure if exists optionalClause;
CREATE PROCEDURE optionalClause()
DELIMITER $$
CREATE DEFINER=`root`@`localhost` PROCEDURE `optionalClause`()
begin
    DECLARE x INT DEFAULT 0;
    /* determining the value of x, for example using select into statement*/

    if @x=1 then
        /* select statement 1 */
    end if;

    if @x=2 then
        /* select statement 2 */
    end if;

END;
$$ DELIMITER ;
CALL optionalClause();

and of course you can have else statement instead of two separate if-statements and make it as complicated as you want. Good Luck!

Answer 3

You could try something along these lines, replace the variable with the correct integer from your select statement.

declare @x int = 2

if @x = 1 goto No1
if @x = 2 goto No2

No1:

print ('This is x = 1')
select * from table where y=3 and z=4;

goto TheEnd

No2:

print ('This is x = 2')
select * from table where y=3 and z=5 and q=6 and r=12;

goto TheEnd

TheEnd:

Print('you made it')

Answer 4

CASE is going to give you IF-THEN logic within a SELECT statement. For example:

SELECT CASE WHEN x=1 THEN field1 ELSE field2 END AS [Conditional Field]
FROM tablename;

Alternatively, you can use IF-THEN-ELSE to run one statement or another. For example: