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This article mainly tells you how to use the sub-query behind mysql select, related technical terms you can check online or find some related books to supplement, here will not dabble, we will go straight to the topic, hope that the sub-query behind mysql select How to use this article can bring you some practical help.

The subquery after select is used and can only be scalar query.

#Case: Query the number of employees in each department

SELECT d.*, ( SELECT COUNT(*) FROM employees WHERE employees.` department_id`=d.department_id)FROM departments d;

#Case: Query department name with employee number =102

SELECT department_nameFROM employees eINNER JOIN departments dON e.` department_id`=d.` department_id`WHERE e.` employee_id`=102;

subquery

SELECT (SELECT department_nameFROM employees eINNER JOIN departments dON e.` department_id`=d.` department_id`WHERE e.` employee_id`=102

);

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