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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.
The performance of a query plan is determined largely by the order in which the tables are joined. For example, when joining 3 tables A, B, C of size 10 rows, 10,000 rows, and 1,000,000 rows, respectively, a query plan that joins B and C first can take several orders-of-magnitude more time to execute than one that joins A and C first.
A right join is employed over the Target (the INTO table) and the Source (the USING table / view / sub-query)--where Target is the left table and Source is the right one. The four possible combinations yield these rules:
An inner join (or join) requires each row in the two joined tables to have matching column values, and is a commonly used join operation in applications but should not be assumed to be the best choice in all situations. Inner join creates a new result table by combining column values of two tables (A and B) based upon the join-predicate.
A derived table is a subquery in a FROM clause. Essentially, the derived table is a subquery that can be selected from or joined to. Derived table functionality allows the user to reference the subquery as a table. The derived table also is referred to as an inline view or a select in from list.
J.D. Power released its 2024 mortgage lender customer satisfaction survey — and the results are surprising. Here are the major changes in lender satisfaction.
Any Texas or Texas A&M player has heard the lore of the rivalry between the two schools, a grudge match that dates to 1894. Third-ranked Texas (10-1, 6-1) and No. 20 Texas A&M (8-3, 5-2) meet ...
Ingres supports joins with hash join, sort-merge join, and nested loop join algorithms. The query optimizer determines which type of join algorithm to use based on its analysis of the query. Nested-loop joins are most often seen on disjoint queries, where correlation variables and table names are arbitrarily used in random order.