Moon subquery SQL

Subquery definition: Using a SELECT statement inside an external SQL statement, this SELECT statement is called a subquery

Sub-query type:

Single-line subquery: Either no results are returned for external SQL, or only one row is returned. Single-row subquery special case: Is this number of queries a column, this seed query is called

"Scalar Quantum Query"

Multiline subquery: Returns one or more rows to an external SQL statement

and sub-query can be divided into:

Dolez query: Returning a queue to an external SQL statement

Associated subquery (correlated Sbuquery): associated with an external SQL statement by the same column (one or more columns referencing an external SQL statement)

Nested subqueries: In another subquery

6.2 Writing a single-line subquery (which can be placed in the WHERE/HAVING/FROM clause)

1. Place in the WHERE clause

Select First_Name, last_name

From customers

where customer_id=

(Select customer_id

From customers

where last_name= "Brown");

This subquery in the WHERE clause executes first, returns the ID of the last_name= "Brown" line, and then passes it to the external WHERE clause to execute the external query.

Use other operators: <,<=,>,>=

For example

Select product_id, Name, price

Form Products

where prict>

(select AVG (Price)

From products)

2 using subqueries in a HAVING clause

The HAVING clause acts to filter a row group and use a subquery in the HAVING clause of an outer query: The HAVING clause returns based on a subquery

The result of filtering the row group

Moon subquery SQL