原文链接:https://www.cnblogs.com/qlqwjy/p/8598091.html
比如在Northwind数据库中有一个查询为
SELECT c.CustomerId,CompanyName FROM Customers c
WHERE EXISTS(
SELECT OrderID FROM Orders o WHERE o.Custom...
比如在Northwind数据库中有一个查询为
SELECT c.CustomerId,CompanyName FROM Customers c
WHERE EXISTS(
SELECT OrderID FROM Orders o WHERE o.CustomerID=c.CustomerID)
这里面的EXISTS是如何运作呢?子查询...
exists关键字介绍
exists强调的是 是否返回结果集,不要求知道返回什么,比如:
SELECT * FROM AM_USER WHERE EXISTS (SELECT 1 FROM AM_ROLE WHERE NAME = 'admin')
只要exists引导的子句有结果集返回,那么exis...
1. 优化原则:小表驱动大表,即小数据集驱动大数据集。
select * from A where id in (select id from B)
等价于:
for select id from B
for select * from A where A.id = B.id
当B表的数据集必须小于A的数据集...
MFA即:Multi-factor authentication (MFA)
今天在为自己账号Activate MFA时报错,如下图所示:
Entity already exists
This entity already exists. Learn more
MFADevice entity at the same path and name al...