What is Subquery?What are the types of Subquery?
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A subquery is a query that is nested inside a SELECT, INSERT, UPDATE, or DELETE statement, or inside another subquery. A subquery can be used anywhere an expression is allowed. In this example a subquery is used as a column expression named MaxUnitPrice in a SELECT statement.
Types of Subqueries
In terms of the placement of the subquery, there are three types:
1. Nested Subquery: The subquery appears in the WHERE clause of the SQL.
2. Inline View: The subquery appears in the FROM clause of the SQL.
3. Scalar Subquery: The subquery appears in the SELECT clause of the SQL.
In terms of the way the subquery is parsed, there are two categories of subqueries:
1. Simple Subquery: This is the kind we saw above. A simple subquery is evaluated once only for each table.
2. Correlated Subquery: This is a type of nested subquery that uses columns from the outer query in its WHERE clause. A correlated subquery is evaluated once for each row.
Types of Subqueries
In terms of the placement of the subquery, there are three types:
1. Nested Subquery: The subquery appears in the WHERE clause of the SQL.
2. Inline View: The subquery appears in the FROM clause of the SQL.
3. Scalar Subquery: The subquery appears in the SELECT clause of the SQL.
In terms of the way the subquery is parsed, there are two categories of subqueries:
1. Simple Subquery: This is the kind we saw above. A simple subquery is evaluated once only for each table.
2. Correlated Subquery: This is a type of nested subquery that uses columns from the outer query in its WHERE clause. A correlated subquery is evaluated once for each row.
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