联合所有使用if/else T-SQL

Question

我在想如何使用IF/ELSE语句来创建一个UNION ALL。

例如：

SELECT * FROM A 
UNION ALL 
SELECT * FROM B 
UNION ALL 

IF @type = 1 
    BEGIN 
     SELECT * FROM C 
    END 
ELSE 
    BEGIN 
     SELECT * FROM D 
    END 

UNION ALL 

SELECT * FROM E 

我得到的语法错误。

Answer 1

SELECT * FROM A 
UNION ALL 
SELECT * FROM B 
UNION ALL 
SELECT * FROM C WHERE @type = 1 
UNION ALL 
SELECT * FROM D WHERE @type <> 1 OR @type IS NULL 
UNION ALL 
SELECT * FROM E ; 

Answer 2

一个好办法做到这一点是使用动态SQL

先建的查询字符串，然后执行它。这样，你有完全的控制

declare query nvarchar(max) 
set query = 'SELECT * FROM A 
      UNION ALL 
      SELECT * FROM B 
      UNION ALL ' 

IF @type = 1 
BEGIN 
set query = query + ' 
      SELECT * FROM C' 
END 
ELSE 
BEGIN 
set query = query + ' 
      SELECT * FROM D' 
END 
set query = 'UNION ALL 
      SELECT * FROM E' 

exec(query)