sql-server - T-SQL 使用 EXISTS 作为列

Question

我想知道我是否可以在这样的列中使用 EXISTS （或类似的东西）：

SELECT Column1,
       Column2,
       EXISTS (SELECT 1 FROM Table2 T2 WHERE T2.Column = T1.Column) AS IsFlag
FROM Table1

我知道我可以用 Count() 做类似的事情

SELECT Column1,
       Column2,
       (SELECT Count(*) FROM Table2 T2 WHERE T2.Column = T1.Column) AS IsFlag
FROM Table1

但是当 Table2 很大时，这可能不是很有效

Answer 1

试试这个

SELECT Column1,
       Column2,
       CASE WHEN EXISTS (SELECT 1 FROM Table2 T2 
       WHERE T2.Column = T1.Column) then 1 ELSE 0 END AS IsFlag
FROM Table1

Answer 2

CASE 
    WHEN 
        EXISTS (SELECT 1 FROM Table2 T2 WHERE T2.Column = T1.Column) 
    THEN 1 
    ELSE 0 
END AS IsFlag