subquery

The SQL SELECT statement returns a result set of records, from one or more tables.A SELECT statement retrieves zero or more rows from one or more database tables or database views. In most applications, SELECT is the most commonly used data manipulation language (DML) command. As SQL is a declarative programming language, SELECT queries specify a result set, but do not specify how to calculate it. The database translates the query into a "query plan" which may vary between executions, database versions and database software. This functionality is called the "query optimizer" as it is responsible for finding the best possible execution plan for the query, within applicable constraints.
The SELECT statement has many optional clauses:

SELECT clause is the list of columns or SQL expressions that must be returned by the query. This is approximately the relational algebra projection operation.
AS optionally provides an alias for each column or expression in the SELECT clause. This is the relational algebra rename operation.
FROM specifies from which table to get the data.
WHERE specifies which rows to retrieve. This is approximately the relational algebra selection operation.
GROUP BY groups rows sharing a property so that an aggregate function can be applied to each group.
HAVING selects among the groups defined by the GROUP BY clause.
ORDER BY specifies how to order the returned rows.

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