Question

如何使用IF EXISTS语句（SQL Server）从子查询中选择布尔值？

应该是这样的：

SELECT 
  TABEL1.Id, 
  NewFiled = (IF EXISTS(SELECT Id FROM TABLE2 WHERE TABLE2.ID = TABEL1.ID) 
                 SELECT 'TRUE' 
              ELSE 
                 SELECT 'FALSE') 
FROM TABEL1

Answer 1

使用CASE：

SELECT 
  TABEL1.Id, 
  CASE WHEN EXISTS (SELECT Id FROM TABLE2 WHERE TABLE2.ID = TABLE1.ID)
       THEN 'TRUE' 
       ELSE 'FALSE'
  END AS NewFiled  
FROM TABLE1

如果TABLE2.ID是唯一身份或主要密钥，您也可以使用此身份：

SELECT 
  TABEL1.Id, 
  CASE WHEN TABLE2.ID IS NOT NULL
       THEN 'TRUE' 
       ELSE 'FALSE'
  END AS NewFiled  
FROM TABLE1
  LEFT JOIN Table2
    ON TABLE2.ID = TABLE1.ID

Answer 2

您还可以使用ISNULL和select语句来获得此结果

SELECT
Table1.ID,
ISNULL((SELECT 'TRUE' FROM TABLE2 WHERE TABLE2.ID = TABEL1.ID),'FALSE') AS columName,
etc
FROM TABLE1

Answer 3

SELECT Id, 'TRUE' AS NewFiled FROM TABEL1
INTERSECT
SELECT Id, 'TRUE' AS NewFiled FROM TABEL2
UNION
SELECT Id, 'FALSE' AS NewFiled FROM TABEL1
EXCEPT
SELECT Id, 'FALSE' AS NewFiled FROM TABEL2;

Answer 4

使用CASE语句并按照以下方式执行操作：

SELECT 
    T1.Id [Id]
    ,CASE WHEN T2.Id IS NOT NULL THEN 'TRUE' ELSE 'FALSE' END [Has Foreign Key in T2]
FROM
    TABLE1 [T1]
    LEFT OUTER JOIN
        TABLE2 [T2]
        ON
        T2.Id = T1.Id

如何使用带有IF EXISTS子查询的SQL Select语句？

4 个答案: