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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:
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 ;
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).
Query optimization is a feature of many relational database management systems and other databases such as NoSQL and graph databases.The query optimizer attempts to determine the most efficient way to execute a given query by considering the possible query plans.
Evaluate Database features and Database related products. Establish and maintain sound backup and recovery policies and procedures. Take care of the Database design and implementation. Implement and maintain database security (create and maintain users and roles, assign privileges). Database tuning and performance monitoring.
The database schema is the structure of a database described in a formal language supported typically by a relational database management system (RDBMS). The term " schema " refers to the organization of data as a blueprint of how the database is constructed (divided into database tables in the case of relational databases ).
For what it's worth, calling a correlated subquery "categorically inefficient", compared to an inline view crunching an entire table, is very inaccurate. If the outer query is in any way bounded, and pulls a subset of rows, using a correlation is preferable to crunching down an entire table within a subquery.