Can we use subquery in SELECT clause?

Can we use subquery in SELECT clause?

You can use subqueries in SELECT, INSERT, UPDATE, and DELETE statements wherever expressions are allowed. For instance, you can use a subquery as one of the column expressions in a SELECT list or as a table expression in the FROM clause. A subquery must include a SELECT clause and a FROM clause.

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.

Does CTE increase performance?

This can result in performance gains. Also, if you have a complicated CTE (subquery) that is used more than once, then storing it in a temporary table will often give a performance boost.

What is the requirement for using a subquery in the SELECT clause?

Subqueries must be enclosed within parentheses. A subquery can have only one column in the SELECT clause, unless multiple columns are in the main query for the subquery to compare its selected columns. An ORDER BY command cannot be used in a subquery, although the main query can use an ORDER BY.

When to use a subquery in the FROM clause?

Subqueries in the FROM clause create a derived or intermediate table that can be used directly to fetch results for the main SELECT query or joined with other tables and then used subsequently.

Is the full SELECT syntax valid in subqueries?

The full SELECT syntax is valid in subqueries. Expression subqueries are used in a query wherever expressions are valid. They return a single value, as opposed to a column or table. Expression subqueries can be correlated. A subquery inside an expression is interpreted as a scalar subquery.

How does a sub query affect the performance of a query?

You’ll note that, even with the compile on each execution, the query using a sub-query actually out-performed the query that was not using a sub-query. The results are even more dramatic when we take away the compile time:

Can a subquery be introduced with a comparison operator?

Subqueries can be introduced with one of the comparison operators (=, < >, >, > =, <, ! >, ! <, or < =). A subquery introduced with an unmodified comparison operator (a comparison operator not followed by ANY or ALL) must return a single value rather than a list of values, like subqueries introduced with IN.