SELECT COUNT 优化为 EXISTS
it1352
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问题说明
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.
正确答案
#1
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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