With a relational database, you can store related data in more than one table. In addition to being able to extract data from related tables using a join, you can also extract it using a subquery. A subquery is a SELECT statement nested within the SELECT, WHERE, or HAVING clause of a parent SQL statement.
Subqueries make some queries easier to write than joins, and there are queries that cannot be written without using subqueries.
Subqueries can be categorized in different ways:
whether they can return one or more rows (single-row vs. multiple-row subqueries)
whether they are correlated or uncorrelated
whether they are nested within another subquery
Single-row and multiple-row subqueries
Correlated and uncorrelated subqueries
Use of subqueries instead of joins
Subqueries in the WHERE clause
Subqueries in the HAVING clause
Optimizer automatic conversion of subqueries to joins
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