If Exists 命令设置变量

Question

我有一个有效的 IF EXISTS 命令，用于 select 表中已有的 PID_GUID，或 select 用作 PID_GUID 的值如果它不存在于表中。命令如下所示；

IF EXISTS (SELECT PID_GUID FROM PID WHERE EDI_ID = '12874' OR PID = 'ROBERT' OR PID = 'R595')
 BEGIN
   SELECT PID_GUID FROM PID WHERE EDI_ID = '12874' OR PID = 'ROBERT' OR PID = 'R595'
 End
 ELSE
   SELECT 'a70600f4-1cff-4284-a2ce-5eb19f47cf19'

现在我想做的是将其放入设置这样的变量中；

Daclare @OLDPID = VARCHAR(36)
SET @OLDPID = IF EXISTS (SELECT PID_GUID FROM PID WHERE EDI_ID = '12874' OR PID = 'ROBERT' OR PID = 'R595')
 BEGIN
   SELECT PID_GUID FROM PID WHERE EDI_ID = '12874' OR PID = 'ROBERT' OR PID = 'R595'
 End
 ELSE
   SELECT 'a70600f4-1cff-4284-a2ce-5eb19f47cf19'

我将如何在 SQL2008 中执行此操作？

Answer 1

在每个语句中设置变量：

IF EXISTS (SELECT PID_GUID FROM PID WHERE EDI_ID = '12874' OR PID = 'ROBERT' OR PID = 'R595')
 BEGIN
   SELECT @OLDPID = PID_GUID
   FROM PID
   WHERE EDI_ID = '12874' OR PID = 'ROBERT' OR PID = 'R595'
 End
 ELSE
   SELECT @OLDPID = 'a70600f4-1cff-4284-a2ce-5eb19f47cf19';

其实我更倾向于使用：

DECLARE @OLDPID VARCHAR(36) = 'a70600f4-1cff-4284-a2ce-5eb19f47cf19';
IF EXISTS (SELECT PID_GUID
           FROM PID
           WHERE EDI_ID = '12874' OR PID = 'ROBERT' OR PID = 'R595'
          )
 BEGIN
     SELECT @OLDPID = PID_GUID
     FROM PID
     WHERE EDI_ID = '12874' OR PID = 'ROBERT' OR PID = 'R595';
 END;

Answer 2

我会使用 COALESCE()，因为 COALESCE() 无所不能。

SELECT @OLDPID = COALESCE((SELECT PID_GUID 
                          FROM PID WHERE EDI_ID = '12874' OR PID = 'ROBERT' OR PID = 'R595'
                          FETCH FIRST 1 ROW ONLY),
                          'a70600f4-1cff-4284-a2ce-5eb19f47cf19')

我们不需要讨厌的 IF 语句！

SQL 服务器 --

SELECT @OLDPID = COALESCE((SELECT TOP 1 PID_GUID 
                          FROM PID WHERE EDI_ID = '12874' OR PID = 'ROBERT' OR PID = 'R595'),
                          'a70600f4-1cff-4284-a2ce-5eb19f47cf19')

If Exists 命令设置变量

If Exists command in setting a variable

sql

sql-server

variables

if-statement

exists