sql check if row exists

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

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

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

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,

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

exists summary of database SQL statements SQL exists in learningFirst to compare the following syntax:--deals= Trading tables, areas= regional tables, such as Hong Kong; Our purpose: To view the regions with transactionsSELECT * from areas where ID

    In MSSQL, inserting a record is simple, but in some special applications, before inserting a record, you need to check whether the record exists. The insert operation is performed only when the record does not exist, this article describes the

Text: The exists in the SQL drip 35-sql statementFor example, in the Northwind database there is a query for select C.customerid,companyname from Customers C where EXISTS (select OrderID from Orders o where O . Customerid=c.customerid) How does this

exists is used to check if a subquery returns at least one row of data, and the subquery does not actually return any data, but instead returns a value of TRUE or FalseEXISTS Specifies a subquery that detects the existence of a row.Instance:(i).