问题描述
我试图找出表中是否存在一行.使用 MySQL，做这样的查询是否更好:
I'm trying to find out if a row exists in a table. Using MySQL, is it better to do a query like this:
SELECT COUNT(*) AS total FROM table1 WHERE ...
并检查总数是否为非零，或者执行这样的查询是否更好:
and check to see if the total is non-zero or is it better to do a query like this:
SELECT * FROM table1 WHERE ... LIMIT 1
并检查是否返回了任何行?
and check to see if any rows were returned?
在两个查询中，WHERE 子句都使用索引.
In both queries, the WHERE clause uses an index.
推荐答案
你也可以试试EXISTS:
SELECT EXISTS(SELECT * FROM table1 WHERE ...)
并且根据 文档，你可以SELECT任何东西.
and per the documentation, you can SELECT anything.
传统上，EXISTS 子查询以 SELECT * 开头，但它可以以 SELECT 5 或 SELECT column1 或任何东西开头.MySQL忽略此类子查询中的 SELECT 列表，因此没有区别.
Traditionally, an EXISTS subquery starts with SELECT *, but it could begin with SELECT 5 or SELECT column1 or anything at all. MySQL ignores the SELECT list in such a subquery, so it makes no difference.
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