SQL查询和错误子查询返回了多个值

Question

Hello here is my SQL query :

WHERE table1.Regnumber IN (
    CASE 
        WHEN @fRegNumber IS NOT NULL AND @fRegNumber<>-1 THEN @fRegNumber
        ELSE (SELECT Regnumber FROM table2)
    END
)

An error:

'Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.'

How can I fix this?

Answer 1

Change to:

WHERE IF(@fRegNumber IS NOT NULL AND @fRegNumber<>-1,
         table1.Regnumber = @fRegnumber),
         table1.Regnumber IN (SELECT Regnumber FROM table2))

WHERE x IN can be used in two ways:

WHERE x IN (expr1, expr2, expr3, ...)

In this case, if you use a subquery as the expression, it must return just a single value. A subquery that returns multiple values will not be spliced in.

or:

WHERE x IN (subquery)

This is the format that allows a subquery to return multiple rows, and x will be tested against all of them.

Since you want to conditionalize whether to test against a single value or the results of a subquery, you must do that outside the IN clause, using IF .

There may be other ways to write this query as a LEFT JOIN.

Answer 2

The problem is with this subquery.

(SELECT Regnumber FROM table2)

It's returning every regnumber from the table. In the context of the overall query, you have to pick just one. The first step in solving your problem is to decide which one you want and why.

Answer 3

I can see 2 approaches here. One keeping the CASE statement this way:

WHERE
    CASE    WHEN    COALESCE(@fRegNumber, -1) == -1
            THEN    t1.regnumber IN (SELECT t2.regnumber FROM table2))
            ELSE    t1.regnumber = @fRegNumber
    END

And the other one rephrasing the whole CASE statement into the appropriate logical operators:

WHERE
  (COALESCE(@fRegNumber, -1) != -1 AND t1.regnumber = @fRegNumber) OR
  (COALESCE(@fRegNumber, -1) == -1 AND t1.regnumber IN (SELECT t2.regnumber FROM table2))