list exists

Source of the blog post (exists and not EXISTS usage in Oracle): http://chenshuai365-163-com.iteye.com/blog/1003247Bowen Source (in VS. exists statement efficiency issues): http://www.cnblogs.com/iceword/archive/2011/02/15/1955337.htmlAexists (SQL

SQL query in the in, exists, not in, not exists usage and difference, sqlexists 1. in and exists In connects the External table to the internal table as a hash (Dictionary set), while exists loops the External table and queries the internal table

Usage and difference of in, exists, not in, and not exists in SQL queries 1. in and exists In connects the External table to the internal table as a hash (Dictionary set), while exists loops the External table and queries the internal table after

Summary:Exists (the result set returned by SQL is true) not exists (the result set returned by SQL is true) is as follows: Table A ID name 1 A1 2 A2 Exists (the result set returned by SQL is true)Not exists (SQL does not return true result sets)As

from the point of view of efficiency: 1 SELECT * from T1 where exists (select 1 from T2 where t1.a=t2.a); When the amount of T1 data is small and the T2 data is very large, the query efficiency of the T1 2) SELECT * from T1 where t1.a in (select t2

EXISTS (SQL returns the result set to true) not EXISTS (SQL does not return the result set to True) is as follows: Table A ID NAME1A12A23A3 Table B ID AID NAME1 1B12 2B23 2B3 table A and table B are 1 pairs of relationships a.id => B.aid

In is the appearance and the inner table as a hash connection, and exists is the external loop loop, each loop loop and then query the internal table.NOT exists: Do NL, subquery first, there is a virtual table, there is a definite value, so even if

Transferred from Https://www.cnblogs.com/liyasong/p/sql_in_exists.html and http://blog.csdn.net/lick4050312/article/details/4476333Table ShowThe two tables involved in the query, a user and an order table, are the following:User table:　　　　Order

For example, there is a query in the Northwind database thatSELECT C.customerid,companyname from Customers CWHERE EXISTS (SELECT OrderID from Orders o WHERE o.customerid=c.customerid)How does this exists work? The subquery returns the OrderID field,

ExistsReturns true or false, depending on whether the result set returned by the subquery contains at least one row. Unlike other predicates (such as in) and logical expressions, exists cannot return unknown. Subquery or return the row set, or do