if else condition in sql

This question already has an answer here:

How do I perform an IF…THEN in an SQL SELECT? 22 answers

Can you have if-then-else logic in SQL? [duplicate] 7 answers

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一种方法是select和case ：

select (case when exists (select X from table1 where x = 'value1')
             then 1
             when exists (select x from table2 where x = 'value2')
             then 2
             else 0
        end) as flag

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Is it possible to implement with variables:

DECLARE @FLAG INT = 0;

SELECT @FLAG = 1 FROM table1 WHERE x = 'value1'

IF @FLAG = 0 
BEGIN
    SELECT @FLAG = 2 FROM table2 WHERE x = 'value2'
END

SELECT @FLAG

The @FLAG variable will hold the value 0, 1 or 2 as the tables contains or not data. If the 1st select does not contain data, then you run the second, if none return data, then return 0 (default @FLAG value).

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这应该工作，虽然它不是一个有效的查询，也不是使用这样的查询的最佳实践。

select 
    case when exists (select X from table1 where x = 'value1') then 1 
    when exists (select x from table2 where x = 'value2') then 2 
    else 0 
end;

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