sql - 选择员工姓名和部门（如果不存在）为部门选择前 1 名员工

Question

我需要从employee条件 onname和的表中获取记录department。如果该名称不存在任何记录并离开，那么它应该从employeefor that返回前 1 条记录department。

前任：

SELECT * FROM EMPLOYEE WHERE NAME = 'SQL' AND DEPID = 1
 IF NOT EXISTS 
     SELECT TOP 1 * FROM EMPLOYEE WHERE DEPID = 1

这可以在单个查询中实现并且还考虑性能。

Answer 1

这应该适用于 MS SQL：

SELECT TOP 1 *
FROM employee
WHERE depid = 1
ORDER BY CASE WHEN name = 'SQL' THEN 0 ELSE 1 END

对于 PostgreSQL 和 MySQL，使用：

SELECT * FROM employee
WHERE depid = 1
ORDER BY name != 'SQL'
LIMIT 1

为获得最佳性能，请在(depid, name).

Answer 2

尝试这个

SELECT TOP 1 * FROM EMPLOYEE WHERE DEPID = 1 AND 
NAME = CASE 
       WHEN EXISTS(SELECT 1 FROM EMPLOYEE WHERE DEPID = 1 AND NAME = 'SQL') 
       THEN 'SQL' ELSE NAME END

Answer 3

我假设您使用的是 SQL 服务器。

一个简单的 If exists else 逻辑将在这里工作。

If exists(SELECT 1 from employee WHERE ename='SQL' AND deptno=1)
SELECT * from employee WHERE name='SQL' AND deptno=1
else 
select top 1 * from empoyee where deptno=1

Answer 4

select top 1 * from employee where depid = 1 and not exists (select * from employee where name = 'SQL' and depid = 1)
union all
select * from employee where name = 'SQL' and depid = 1