A subquery is a query inside another SQL query. It usually appears in SELECT, INSERT, UPDATE or DELETE statements and is surrounded by brackets; it can be nested in multiple locations of the main query, such as WHERE, FROM or SELECT clauses, and is used to provide data conditions or result sets; depending on the returned results, it can be divided into scalar subqueries (one row and one column), row subqueries (one row and multiple columns), table subqueries (multiple row and multiple columns), and existence subqueries (EXISTS); compared with JOIN, subqueries are suitable for filtering or calculating before querying, while JOIN is more suitable for obtaining combined data from multiple tables; when using it, you need to pay attention to: subqueries must be wrapped in brackets to avoid excessive nesting affecting readability and performance, pay attention to field scope, and deal with possible null values.
Simply put, a subquery is a query inside another SQL query. It usually appears in SELECT, INSERT, UPDATE, or DELETE statements and is surrounded by brackets.
Basic usage of subquery
Subqueries can be nested in multiple locations in the main query, such as in the WHERE clause, the FROM clause, or the SELECT clause. Its function is usually to provide data conditions or result sets for external queries.
For example:
Suppose you have two tables, one is employees , which records employee information; the other is departments , which records department information. You want to find employees whose salary is higher than the average salary of the R&D department:
SELECT name, salary
FROM employees
WHERE salary > (
SELECT AVG(salary)
FROM employees
WHERE department_id = (
SELECT id
FROM departments
WHERE name = 'R&D Department'
)
);In this example, the innermost subquery is used to find the department ID of the "R&D Department", the middle subquery calculates the average salary of the department, and the outer layer query filters out employees whose salary is higher than this average.
Common types of subqueries
Depending on the returned results, subqueries can be divided into the following categories:
- Scalar subquery : Returns only one row and one column, that is, a single value.
- Row subquery : Returns a row, but may contain multiple columns.
- Table subquery : Returns multiple rows and multiple columns, like a temporary table.
- Existence Subquery (EXISTS) : used to determine whether there is data that meets the criteria.
Different usage scenarios determine which type of subquery you should choose.
The difference between subquery and JOIN
Many people will be confused about which is better, subquery or JOIN. In fact, they have their own applicable occasions:
- If you need to get data from multiple tables and combine them into a result set, JOIN is more suitable.
- If you need to do filtering or calculation first, and then conduct further query based on this result, subquery is more convenient.
In terms of performance, sometimes JOIN is more efficient than subquery, especially when the data volume is large. However, modern database optimizers are already very smart, and the execution plan of the two is the same in many cases.
Things to note when using subqueries
- Subqueries must be placed in brackets.
- Don't nest too many layers, otherwise it will affect readability and performance.
- Pay attention to the scope of fields. Some fields cannot be accessed directly in subqueries.
- If the subquery may return a null value, remember to consider the impact of this situation on the main query.
For example:
SELECT name
FROM employees
WHERE department_id = (
SELECT id
FROM departments
WHERE name = 'Doesn't exist'
);If the "Disexual Department" does not have a corresponding ID, the subquery returns NULL and the final result will be empty. In this case, it is best to add some judgment logic to avoid unexpected situations.
Basically that's it. Subquery is a very practical feature. Mastering it well allows you to write more flexible and powerful SQL queries.
The above is the detailed content of what is a subquery in mysql. For more information, please follow other related articles on the PHP Chinese website!
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