Question

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

以上查询无法正常工作，因为参数有时可能是空的。

Answer 1

=运算符不支持通配符。您应该使用like运算符：

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

或者，您可以使用or运算符将其短路：

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 IS NULL OR customer_name = @customer)

Answer 2

当参数不确定有值时使用

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 like
case 
when @customer is not null 
then @customer 
else 
'%'
end

用于空参数的情况

2 个答案: