在SQL的SELECT中实现循环查找、双层和多层循环(迭代)查找 SQL如何实现编程语言的for循环查询 MySQL的Select子查询

MySQL错误1093深度解析:UPDATE子查询如何正确改写 在MySQL数据库开发中,更新操作偶尔会遭遇报错,其中错误1093常让开发者困惑。该错误源于在同一UPDATE语句的子查询中直接引用了目标表,违反了MySQL对表读写一致性及锁机制的安全约束。理解此原理后,可通过派生表双层包装、JOIN改写或临时表等方案优雅解决。尤其在数据排名回填、分组极值更新等业务场景里,掌握这些改写技巧能显著提升SQL健壮性。本文以错误1093为切入点,系统讲解其成因与主流修复方案,帮助你快速定位并处理此类更新异常。 阅读详情

在SQL的SELECT中实现循环查找、双层和多层循环(迭代)查找 SQL如何实现编程语言的for循环查询 MySQL的Select子查询。

一、背景

在查询表极大,可能需要使用特定的查询索引的时候,使用SELECT子查询会比使用Inner Join查询效率要高。

如,我需要知道美国站点的在线listing数。普通的查询语句,直接使用Inner Join查询会由于店铺数量出现极大的查询性能问题。若店铺少,可能会几秒,随着店铺多起来,查询等待时间可能成百上千倍的增长,而不是预期的线性增长。甚至很多时候,会直接卡死。(原表上亿行)。

使用循环式查找则是将店铺表作为主表,然后嵌套SELECT子查询来实现。有点类似于每个店铺都是用编程语言的for循环,来逐条执行。这种方式快速且稳定。当表格极大,且需要用到特定的索引时,查询性能提升尤为明显。(注意,这里店铺表才几百个店铺,而listing有上亿)

二、一层迭代(一层循环)

①需求:查找以深圳开头、国家为美国的店铺在平台的美国站点的在线listing数。

②、Inner Join实现

/*
普通Inner Join。

时间:45.067
*/
select
    os.OrderSourceID
    , count(1) listing表在线listing数
from sys_ordersource os 
inner join m_ebay_listing el on el.OrderSourceId=os.OrderSourceID
where os.CustomerID=1
    and os.OrderSourceType=2
    and os.OrderSourceCountry = 'US'
    and os.OrderSourceName like '深圳%'
    and el.SiteCode='US'
    an
Mysql 查询案例--基础查询100道+进阶查询100道 阅读详情

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