Order by 与union all优先级

在 SQL 查询语句中，ORDER BY 和 UNION ALL 的优先级是不同的。具体来说，ORDER BY 的优先级比UNION ALL高。

这意味着，当查询中同时包含ORDER BY 和UNION ALL 时，ORDER BY 会先执行，然后才执行UNION ALL。因此，如果你想在UNION ALL之前对结果进行排序，则应该将ORDER BY 子句放在UNION ALL之前。

例如，以下查询首先按照salary字段对employee表进行排序，然后再将结果与department表合并：

SELECT employee.name, employee.salary, department.department_name
FROM employee
INNER JOIN department ON employee.department_id = department.department_id
ORDER BY employee.salary
UNION ALL
SELECT employee.name, employee.salary, department.department_name
FROM employee
INNER JOIN department ON employee.department_id = department.department_id
ORDER BY department.department_name;

在这个查询中，首先对employee表按照salary字段进行排序，然后使用UNION ALL将结果与department表合并。如果我们想按照department_name字段对结果进行排序，则需要在UNION ALL之前加上ORDER BY子句：

SELECT employee.name, employee.salary, department.department_name
FROM employee
INNER JOIN department ON employee.department_id = department.department_id
ORDER BY employee.salary
UNION ALL
SELECT employee.name, employee.salary, department.department_name
FROM employee
INNER JOIN department ON employee.department_id = department.department_id
ORDER BY department.department_name
ORDER BY department.department_name;

在这个查询中，首先对employee表按照salary字段进行排序，然后使用UNION ALL将结果与department表合并，最后再按照department_name字段进行排序。