Is subquery better than JOIN?
The good thing in sub-queries is that they are more readable than JOIN s: that’s why most new SQL people prefer them; it is the easy way; but when it comes to performance, JOINS are better in most cases even though they are not hard to read too. Use EXPLAIN to see how your database executes the query on your data.
Under what circumstances would you use a subquery?
A subquery is used to return data that will be used in the main query as a condition to further restrict the data to be retrieved. Subqueries can be used with the SELECT, INSERT, UPDATE, and DELETE statements along with the operators like =, <, >, >=, <=, IN, BETWEEN, etc.
Which is better subqueries or joins in SQL Server?
Almost all SELECT statements that join tables and use the join operator can be rewritten as subqueries, and vice versa. Writing the SELECT statement using the join operator is often easier to read and understand and can also help the SQL Server Database Engine to find a more efficient strategy for retrieving the appropriate data.
How are subqueries nested in a statement in SQL Server?
SQL Server implicitly qualifies the column in the subquery with the table name in the outer query. A subquery can itself include one or more subqueries. Any number of subqueries can be nested in a statement. The following query finds the names of employees who are also sales persons. Here is the result set.
When do subqueries become correlated in SQL Server?
Once a column from the outer query is referenced in the subquery, it becomes a correlated subquery. One more point to consider: if you highlight a subquery and execute it, an error will occur. And yes, you are absolutely right: this makes correlated subqueries pretty harder to debug.
How to execute a subquery in SQL Server?
To perform it, follow these steps: 1 Right-click the execution plan of the statement using the subquery. 2 Select Save Execution Plan As. 3 Name the file subquery-execution-plan.sqlplan. 4 Go to the execution plan of the statement using a join and right-click it. 5 Select Compare Showplan. 6 Select the filename you saved in #3.