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In a SQL database query, a correlated subquery (also known as a synchronized subquery) is a subquery (a query nested inside another query) that uses values from the outer query. This can have major impact on performance because the correlated subquery might get recomputed every time for each row of the outer query is processed.
A nested query is also known as a subquery. While joins and other table operations provide computationally superior (i.e. faster) alternatives in many cases, the use of subqueries introduces a hierarchy in execution that can be useful or necessary. In the following example, the aggregation function AVG receives as input the result of a subquery:
A subquery can use values from the outer query, in which case it is known as a correlated subquery. Since 1999 the SQL standard allows WITH clauses, i.e. named subqueries often called common table expressions (named and designed after the IBM DB2 version 2 implementation; Oracle calls these subquery factoring).
An example of a recursive query computing the factorial of numbers from 0 to 9 is the following: WITH recursive temp ( n , fact ) AS ( SELECT 0 , 1 -- Initial Subquery UNION ALL SELECT n + 1 , ( n + 1 ) * fact FROM temp WHERE n < 9 -- Recursive Subquery ) SELECT * FROM temp ;
Notably, this excludes the use of aggregation and subqueries. For example, the above query can be written as an SQL query of the conjunctive query fragment as select l . student , l . address from attends a1 , gender g1 , attends a2 , gender g2 , lives l where a1 . student = g1 . student and a2 . student = g2 . student and l . student = g1 ...
SQL was initially developed at IBM by Donald D. Chamberlin and Raymond F. Boyce after learning about the relational model from Edgar F. Codd [12] in the early 1970s. [13] This version, initially called SEQUEL (Structured English Query Language), was designed to manipulate and retrieve data stored in IBM's original quasirelational database management system, System R, which a group at IBM San ...
Articles with example SQL code (28 P) C. SQL clients (3 P) M. ... SQLite (1 C, 4 P) Pages in category "SQL" ... (SQL) Correlated subquery; Cubes (OLAP server) Cursor ...
Delete (SQL) 14 languages ... Delete rows from mytable using a subquery in the where condition: ... (this is an example of a many-to-many relationship).