A subquery is a query statement nested in another SQL query and is often used to dynamically obtain values or sets of values for use by the main query. It can appear in SELECT, FROM, WHERE or HAVING clauses, and is most commonly compared in WHERE conditions, such as finding employees whose salary is higher than the average salary; subqueries can also be used to generate derivative tables, as field expressions, or perform existence checks; performance issues should be paid attention to when using them, especially the repeated execution of related subqueries may affect efficiency; at the same time, to ensure that the return result type matches, a single-value comparison needs to return a value, and multiple-value comparisons can be handled by operators such as IN and EXISTS; appropriate use of alias and brackets can improve readability and avoid syntax errors; in some cases, JOIN operations can replace subqueries for better performance and clarity.

Subqueries or nested queries are a common query method in SQL. Simply put, one query contains another query. The outer layer is called the main query, and the inner layer is called the subquery. The results of the subquery are usually used by the main query for further filtering or calculation.

What is a subquery?

A subquery is a query statement nested in another SQL query. It can appear in SELECT, FROM, WHERE, or HAVING clauses. The most common usage is to dynamically obtain a certain value or a set of values in the WHERE condition.

For example: If you want to find employees whose salary is higher than the average salary, you can first write a sub-query to calculate the average salary, and then use this result in the main query for comparison.

 SELECT name, salary
FROM employees
WHERE salary > (SELECT AVG(salary) FROM employees);

In this example, SELECT AVG(salary) in brackets is a subquery.

Common usage scenarios for subquery

1. Filter data : For example, find employees who are more than the average salary of a certain department.
2. Generate derived tables : Use subqueries as part of the FROM for temporary tables.
3. Field expression : Use a subquery to return a single value after SELECT, such as counting the order quantity of each customer.
4. Existence check : Use with EXISTS to determine whether there is data that meets the conditions.

The core logic of these scenarios is "first execute the sub-query to get the results, and then use the results to run the main query."

Things to note when using subqueries

• Performance issues : In some cases, especially when subqueries are repeatedly executed, it will affect efficiency. For example, in WHERE, correlated subquery is used, and each time the main query process a line, the subquery must be run again.
• The return result type needs to match : for example, if you use a subquery to get a value for comparison, then it must return only one value, otherwise an error will be reported.
• Use alias appropriately : Especially when a subquery is part of the FROM, it is best to give it an alias, which makes it easier to reference.
• You can consider using JOIN instead : Sometimes the writing of JOIN is clearer and the performance is better, especially in the case of large data volumes.

Actual writing tips

• If the subquery only returns one row and one column (scalar subquery), you can directly participate in the operation, such as the average salary example mentioned above.
• If you return multiple rows and one column (row set subquery), you can use IN, NOT IN, EXISTS and other operators to handle it.
• Subqueries that return multiple rows and columns are generally used as derivative tables, remember to add an alias.
• Sometimes you need to add brackets to avoid syntax errors.

In general, subquery is a very practical function in SQL, which can help us write more flexible and logical query statements. Although it may be a bit confusing to write, it is actually quite easy to use after understanding the execution order and result structure.

Basically that's all, you will become familiar with it if you use it too much.

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