Question

我有两个SELECT查询。

SELECT FullName FROM TableA
SELECT FirstName FROM  TableB

我想在CASE中执行两条语句，或者是否有其他选择。

如果SELECT FullName FROM TableA返回NULL或根本没有任何行，则它应运行以下查询：

SELECT FirstName FROM TableB

我可以在这里使用两个CASE语句，但是每个表中大约有20列，这将使40 CASE语句最终使查询很长。

Answer 1

SELECT FullName FROM TableA
union 
SELECT FirstName FROM  TableB 
where not exists 
  (SELECT FullName FROM TableA)

Working Fiddle

Answer 2

我认为这就是您要搜索的内容

IF EXISTS (SELECT 1 FROM TableA) 
    SELECT FullName AS Name, TableA.Field1 AS FinalField1, 
           TableA.Field2 AS FinalField2... 
    FROM TableA
ELSE
    SELECT FirstName AS Name, TableB.Field1 AS FinalField1, 
           TableB.Field2 AS FinalField2... 
    FROM TableB

如何在CASE中执行SELECT语句

2 个答案: