mysql 自定义函数 case when[通俗易懂]

#字符串局部替换函数

DELIMITER $$
CREATE FUNCTION replaceString(s VARCHAR(14),f VARCHAR(5),r VARCHAR(5)) RETURNS  VARCHAR(14)
BEGIN
		DECLARE findpos INT;
		DECLARE returnstring VARCHAR(14);
		SET findpos=INSTR(s,f);
		IF(findpos>0) THEN 
			SET returnstring=INSERT(s,findpos,LENGTH(f),r);
		ELSE
			SET returnstring=s;
		END IF;
	        RETURN returnstring;
END$$
SELECT replaceString("abc","a","y");

#查询第nth 大的工资

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN
        set n=N-1;
  RETURN (
      # Write your MySQL query statement below.
      select case when(count(Salary)>n) then(select  distinct Salary from Employee order by Salary desc limit n,1) else null end from Employee
  );
END