select case when if

select case when if 的一些用法 - 马丁传奇 - 博客园 https://www.cnblogs.com/martinzhang/p/3220595.html

Write a SQL query to get the second highest salary from the Employee table.

+----+--------+
| Id | Salary |
+----+--------+
| 1  | 100    |
| 2  | 200    |
| 3  | 300    |
+----+--------+

For example, given the above Employee table, the query should return 200 as the second highest salary. If there is no second highest salary, then the query should return null.

+---------------------+
| SecondHighestSalary |
+---------------------+
| 200                 |
+---------------------+

IF (COUNT(Salary)=0 ,"null-str",Salary) AS SecondHighestSalary

if  count  null

SELECT
	CASE COUNT(1)
WHEN 1 THEN
	null
ELSE
	CAST((
		SELECT
			Salary
		FROM
			Employee
		GROUP BY
			Salary
		ORDER BY
			Salary DESC
		LIMIT 1,
		1
	) AS SIGNED INT )
END AS SecondHighestSalary
FROM
	(
		SELECT
			COUNT(1) AS c,
			Salary AS SecondHighestSalary
		FROM
			Employee
		GROUP BY
			Salary
		ORDER BY
			Salary DESC
	) AS t;