sql subquery

SQL Optimization-logical Optimization-subquery optimization (MySQL) and sqlmysql 1) subquery concept:When a query is a subquery of another query, it is called a subquery (a query statement is nested in the query statement ). Subqueries are located: A) Target column posi

col2, but it generates up to one output for each tab1 row, even if there are multiple matching tab2 rows: SELECT col1 from Tab1 where EXISTS (SELECT 1 from tab2 WHERE col2 = tab1.col2); 2.in expression in (subquery) The right-hand side of this form of in is a parenthesized subquery that must return only one field. The left-hand-side expression evaluates and compares each row of the result of the

Technical preparationThe database version is SQL Server2008R2 and is analyzed using Microsoft's simpler case Library (Northwind.1. Independent subqueryThe so-called independent subquery method means that the subquery is irrelevant to the primary query. The advantage of this method is that the subquery does not depe

Original: SQL Server Tuning Series Basics (subquery operations Summary)ObjectiveIn the previous articles, we introduced a series of descriptions of operators and the optimization methods and techniques for each operator. This includes a series of our common operators, including how to view execution plans, how several datasets are commonly used, how to join operators, and parallel operators. Interested chil

subquery results. Also note that the all and any operators cannot be used alone, and can only be used in conjunction with single-line comparators (=, >, Cases:1). Multi-row subqueries use the in operation symbol Example: The query took the name of the student whose teacher was named Rona (assuming only)Sql> Select StnameFrom Studentwhere Stid in (SelectDistinct Stid from score where teid= (select Teid from

ObjectiveIn the previous articles, we introduced a series of descriptions of operators and the optimization methods and techniques for each operator. This includes a series of our common operators, including how to view execution plans, how several datasets are commonly used, how to join operators, and parallel operators. Interested children's shoes can be clicked to view.In this article we introduce a series of sub-query statements, sub-query is generally the formation of some of the basic oper

Multi-table join: You can specify multiple tables directly after the FROM clause. in semantics, you can retrieve data FROM the Descartes of these tables. You can use the WHERE clause to set filtering conditions. The SQL Server link syntax is as follows: SELECT table_name.column_name, table_name.column_name ,...... FROM {table_name, table_name ,......} WHERE table_name.column_name join_operator table_name.column_name In this syntax, the FROM clause li

ObjectiveIn the previous articles, we introduced a series of descriptions of operators and the optimization methods and techniques for each operator. This includes a series of our common operators, including how to view execution plans, how several datasets are commonly used, how to join operators, and parallel operators. Interested children's shoes can be clicked to view.In this article we introduce a series of sub-query statements, sub-query is generally the formation of some of the basic oper

SQL subquery instance SQL subquery instance introduction: A subquery is a query within a query. The results of the subquery are used by the DBMS to determine the results of the advanced query that contains the

Instance analysis SQL nested subqueries: some junior programmers often use nested subqueries (subqueries include a subquery) in SQL syntax, this article provides a basic explanation, and I believe that new users will gain some benefits. Use the subquery Principle 1. A subquery