Subqueries in sqlserver

• Subquery

A subquery is nested in another statement ( Such as ：[SELECT],[INSERT],[UPDATE] or [DELETE]) Query in , It is an application of grammar .

We might as well understand it through examples , The following statement shows how to SELECT Of the statement [WHERE] Clause to find the location in New York ( New York ) Customer's sales order ：

• nested subqueries

Nested subquery means that one subquery can be nested in another subquery .

stay SQL Server in , Most support 32 Nested levels .

give an example ： Find a price higher than “ Shanghai forever ” and “ Phoenix ” The average price of all products of the brand .

For such questions , We need to examine the subject , And make it clear which one needs to be queried in the end ,

First step , First name the brand “ Shanghai forever ” and “ Phoenix ” Of id obtain

The second step , Calculate the average price of all products belonging to two brands

The third step , Use nested subqueries to set prices above “ Shanghai forever ” and “ Phoenix ” The average price of all the products of the brand can be found

Use nested subqueries to simplify query statements and avoid tedious repetition , Is it convenient . The results will not be shown here , You can also try to operate it .

• Correlation subquery

Related subqueries use the values of external queries [ Subquery ]. let me put it another way , It depends on the value of the external query .

Because of this dependency , Related subqueries cannot be executed independently as simple subqueries . Besides , Repeat the related sub query for each row of the external query evaluation . therefore , Related subqueries are also called duplicate subqueries .

give an example ： Find a product whose price is equal to the highest price of its category .

• Exists Operator

1. EXISTS The operator is a logical operator , Used to check whether the subquery returns any rows . If the subquery returns one or more rows , be EXISTS Operator return TRUE .
2.   Here are SQL Server EXISTS The syntax of the operator ：

In this grammar , The subquery is just SELECT sentence . After the sub query returns rows , EXISTS Operator return TRUE And immediately stop processing . Please note that , Even if the subquery returns NULL value , EXISTS The operator also evaluates to TRUE

1. With query EXISTS return NULL Example

give an example ： The following example returns customers All the rows in the table ：

In this example , The subquery returns a query containing NULL The result set , This has also led to EXISTS Operator evaluates to TRUE .

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