将 SELECT COUNT 优化为 EXISTS

Optimizing SELECT COUNT to EXISTS(将 SELECT COUNT 优化为 EXISTS)

问题描述

我有一个查询要从表中查找某些客户.

I have a query to find certain customers from a table.

SELECT COUNT(*)
  FROM CUSTOMER
 WHERE amount <> 0
   AND customerid = 22

customerid 上有一个索引，所以数据库会扫描 customerid = 22 的所有行.

There is an index on customerid, so the DB scans all rows with customerid = 22.

由于结果是通过检查计数返回零还是大于零来处理的，我该如何优化查询?IE.这样，在金额 <> 0 的第一个客户行，查询返回 0，否则如果所有行都 = 0，则返回 1.

Since the result is processed by checking whether the count returns zero or more than zero, how can I optimize the query? I.e. such that at the first customer row with amount <> 0 the query returns 0 else if all rows are = 0, then return 1.

推荐答案

select case
         when exists (select *
                      from   customer
                      where  amount <> 0
                             and customerid = 22) then 1
         else 0
       end  as non_zero_exists

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