-->在含有子查询的SQL语句中,要特别注意减少对表的查询 -->低效: SELECT * FROM employees WHERE department_id = (SELECT department_id FROM departments WHERE department_name = 'Marketing') AND manager_id = (SELECT manager_id FROM departments WHERE department_name = 'Marketing'); -->高效: SELECT * FROM employees WHERE ( department_id, manager_id ) = (SELECT department_id, manager_id FROM departments WHERE department_name = 'Marketing') -->类似更新多列的情形 -->低效: UPDATE employees SET job_id = ( SELECT MAX( job_id ) FROM jobs ), salary = ( SELECT AVG( min_salary ) FROM jobs ) WHERE department_id = 10; -->高效: UPDATE employees SET ( job_id, salary ) = ( SELECT MAX( job_id ), AVG( min_salary ) FROM jobs ) WHERE department_id = 10;
sql 最小化表查询次(col1,col2) = (select col1,col2 from t)
最小化表查询次数
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