Sql 空参数时的情况

上面的查询没有正常工作，

---

as参数有时可能为空

运算符不支持通配符。您应该改用

like

操作符：

declare @customer varchar(50)

select cateogry, product_Name 
from product 
join order 
on product.id = order.product_id
join customer 
on order.customer_id = cutstomer.id 
where category = 'Tea'
and customer_name =
case 
when @customer is not null 
then @customer 
else 
'%'
end 

或者，您可以使用

或
操作员将其短路：

SELECT category, product_Name 
FROM   product 
JOIN   order ON product.id = order.product_id
JOIN   customer ON order.customer_id = cutstomer.id 
WHERE  category = 'Tea' AND
       customer_name LIKE -- Here!
       CASE WHEN @customer IS NOT NULL THEN @customer 
       ELSE '%'
       END

=
运算符不支持通配符。您应该改用
like
操作符：

declare @customer varchar(50)

select cateogry, product_Name 
from product 
join order 
on product.id = order.product_id
join customer 
on order.customer_id = cutstomer.id 
where category = 'Tea'
and customer_name =
case 
when @customer is not null 
then @customer 
else 
'%'
end 

或者，您可以使用
或
操作员将其短路：

SELECT category, product_Name 
FROM   product 
JOIN   order ON product.id = order.product_id
JOIN   customer ON order.customer_id = cutstomer.id 
WHERE  category = 'Tea' AND
       customer_name LIKE -- Here!
       CASE WHEN @customer IS NOT NULL THEN @customer 
       ELSE '%'
       END

当参数不确定是否有值时使用like

SELECT category, product_Name 
FROM   product 
JOIN   order ON product.id = order.product_id
JOIN   customer ON order.customer_id = cutstomer.id 
WHERE  category = 'Tea' AND
       (@customer IS NULL OR customer_name = @customer)

当参数不确定是否有值时使用like

SELECT category, product_Name 
FROM   product 
JOIN   order ON product.id = order.product_id
JOIN   customer ON order.customer_id = cutstomer.id 
WHERE  category = 'Tea' AND
       (@customer IS NULL OR customer_name = @customer)

这当然有帮助。有些行返回比空数据集好。这当然有帮助。有些行返回的数据比空数据集好，这是非标准SQL。您使用的是哪种数据库管理系统？这是非标准SQL。您使用的是哪种数据库管理系统？