What is a subquery in SQL give some use case scenarios?
A subquery, or nested query, is a query placed within another SQL query. However, you can also use subqueries when updating the database (i.e. in INSERT , UPDATE , and DELETE statements). There are several types of SQL subqueries: Scalar subqueries return a single value, or exactly one row and exactly one column.
Which is faster joins or subqueries?
A general rule is that joins are faster in most cases (99%). The more data tables have, the subqueries are slower. The less data tables have, the subqueries have equivalent speed as joins. The subqueries are simpler, easier to understand, and easier to read.
WHERE subqueries can not be used?
4. Where subqueries can not be used? Explanation: The WHERE clause only in the SELECT statement.
Do subqueries reduce performance?
Disadvantages of Subquery: The optimizer is more mature for MYSQL for joins than for subqueries, so in many cases a statement that uses a subquery can be executed more efficiently if you rewrite it as join.
How often do you use subqueries in SQL?
Use subqueries when the result that you want requires more than one query and each subquery provides a subset of the table involved in the query. If a membership question is asked, then a subquery is usually used.
How many subqueries can be written in SQL?
32 levels
Up to 32 levels of nesting is possible, although the limit varies based on available memory and the complexity of other expressions in the query. Individual queries may not support nesting up to 32 levels. A subquery can appear anywhere an expression can be used, if it returns a single value.
What types of joins are used in writing subqueries?
Santhoshkandula
- Santhoshkandula. Answered On : May 11th, 2011.
- There are 5 types of joins. Those are 1. Equi Join / Inner Join 2. Non-Equi Join 3. Outer Join ( Left Outer Join, Right Outer Join, Full Outer Join ) 4. Self Join 5. Cross Join.
What is difference between subquery and correlated subquery?
Sub-query. The inner query execute first, the outer query deprnds on inner query output. Correlated sub-query. The outer query execute first, the inner query deprnds on outer query result. A correlated sub-query is where the sub-query part makes reference to values from the outer query.
What is subselect in DB2?
Db2 11.5. subselect. The subselect is a component of the fullselect. select-clause from-clause where-clause group-by-clause having-clause order-by-clause offset-clause fetch-clause isolation-clause. A subselect specifies a result table derived from the tables, views or nicknames identified in the FROM clause. The derivation can be described as
How to use subquery in join operation in MySQL?
Qualifying column names in subqueries. In the following example,the BusinessEntityID column in the WHERE clause of the outer query is implicitly qualified by the table name in the outer