sql - SQL 查询中的动态 WHERE 子句

Question

我想执行这个查询

select * from locations where region=1 or region=2

但在某些情况下，我不想在 WHERE ( or region=2) 中包含第二个子句。有没有办法有条件地运行 or 语句？

Answer 1

检查 where 语句中的条件

select * from locations 
where region=1 
or (region=2 and @checkRegion2=1)

Answer 2

如果您想要通用查询，您可以为此声明表变量（我假设您使用的是 SQL 服务器）：

declare @regions table (id int primary key)

然后根据条件填写：

insert into @regions select 1
if @use2 = 1
    insert into @regions select 2

现在您可以运行此查询：

select *
from location as l
where exists (select * from @regions as r where r.id = l.region)

或者

select *
from location as l
where l.region in (select r.id from @regions as r)

Answer 3

DECLARE @RUNAFTERBIT VARCHAR(10)
SET @RUNAFTERBIT = 'TRUE'

IF @RUNAFTERBIT = 'TRUE'
  SELECT * FROM LOCATIONS WHERE REGION = 1 OR REGION = 2
ELSE
  SELECT * FROM LOCATIONS WHERE REGION = 1 
END IF

Answer 4

您可以使用动态 SQL：

DECLARE @Query VARCHAR(MAX) = 'SELECT * FROM locations WHERE region=1 '

IF(@RUNAFTERBIT = 'TRUE')
BEGIN
    SET @Query = @Query + ' OR REGION = 2'
END

EXEC(@Query)

Answer 5

在CASE某些情况下，表达式可用于执行短路评估：

declare @AlternateRegion as Int = 2;

select *
  from Locations
  where case
    when Region = 1 then 1
    when @AlternateRegion is not NULL then
      case when Region = @AlternateRegion then 1 else 0 end
    else 0
    end = 1

CASE在存在聚合函数的情况下不能可靠地提供短路评估。请参阅CASE / COALESCE 并不总是按文本顺序进行评估， 并且聚合不遵循 CASE 的语义 。