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SQL or Structured Query Language, as the name suggests, is a language used to query or operate on a database. A powerful aspect of SQL is the use of subqueries, also referred to as inner queries or nested queries. A subquery is a SQL query embedded within a larger SQL query, and typically, they are enclosed within parentheses.
Why Use SQL Subqueries?
Subqueries can be a powerful tool when you need to manipulate data in your database, as they allow you to utilize data from one table for operations on another table. They can be used to retrieve, update and delete data in more complex ways, allowing you to perform dynamic, robust data manipulations all in one step.
Example of a SQL subquery
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SELECT EmployeeId, FirstName, LastName FROM Employees WHERE EmployeeId IN ( SELECT EmployeeId FROM Sales WHERE SaleAmount > 500); |
In the above example, the subquery is the part within the parentheses. It’s selecting the IDs of employees who have made sales above 500. This list of IDs is then used by the main query to retrieve the corresponding employee details.
Types of SQL Subqueries
Single-row subquery
These are the subqueries that return only a single row from the inner SELECT statement.
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SELECT column_name, column_name FROM table1 WHERE column_name OPERATOR ( SELECT column_name FROM table2 WHERE condition ); |
Multiple-row subquery
These are the subqueries that return more than one row from the inner SELECT statement.
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SELECT column_name, column_name FROM table1 WHERE column_name OPERATOR ( SELECT column_name FROM table2 WHERE condition ); |
There’s much more to SQL subqueries, involving more complex SQL queries that use subqueries in FROM, SELECT, and JOIN clauses or subqueries that call from other subqueries. The potential with subqueries is vast and they are a powerful feature that can greatly enhance the performance and flexibility of your SQL code.
Conclusion
Subqueries provide a powerful and flexible tool for developers and database administrators. With the right practice and understanding, one can truly unlock the potential of SQL subqueries to yield effective and efficient results.