Question

子查询返回的值多于一个。当子查询跟随在 =、!=、<、<=、>、>= 之后，或子查询用作表达式时，这种情况是

create TRIGGER [dbo].[BudgetDelete]

on [dbo].[M_Budget_Day]
after delete
as
begin
declare @BudgetYear int ,@ItemGroup varchar(10),@company varchar(10),@month int ,@count decimal(18,4)
set @month = (select month from deleted )
set @BudgetYear = (select budgetYear from deleted)
set @ItemGroup = (select ItemGroup from deleted)
set @company = (select company from deleted)
set @count = 0;
if @month = 1
update M_Budget set Month1 = @count where BudgetYear = @BudgetYear and Company = @company and ItemGroup = @ItemGroup
else if @month = 2
update M_Budget set Month2 = @count where BudgetYear = @BudgetYear and Company = @company and ItemGroup = @ItemGroup
else if @month = 3
update M_Budget set Month3 = @count where BudgetYear = @BudgetYear and Company = @company and ItemGroup = @ItemGroup
else if @month = 4
update M_Budget set Month4 = @count where BudgetYear = @BudgetYear and Company = @company and ItemGroup = @ItemGroup
else if @month = 5
update M_Budget set Month5 = @count where BudgetYear = @BudgetYear and Company = @company and ItemGroup = @ItemGroup

end
GO
出现错误：
子查询返回的值多于一个。当子查询跟随在 =、!=、<、<=、>、>= 之后，或子查询用作表达式时，这种情况是不允许的。

Answer 1

子查询返回值多于一个时只能用in、not in、exists、not exists这些集合操作，=、!=、<、<=、>、>=这些都是单值比较操作符，如果想让子查询返回一个值时可以用sum、max、min等聚集函数

Answer 2

set @month = (select month from deleted )
set @BudgetYear = (select budgetYear from deleted)
set @ItemGroup = (select ItemGroup from deleted)
set @company = (select company from deleted)

这4句可能会有问题，加上一个top 1 就不会有错了。