需求

建表语句

CREATE TABLE Greatests
(key1 CHAR(1) PRIMARY KEY,x   INTEGER NOT NULL,y   INTEGER NOT NULL,z   INTEGER NOT NULL);INSERT INTO Greatests VALUES('A', 1, 2, 3);
INSERT INTO Greatests VALUES('B', 5, 5, 2);
INSERT INTO Greatests VALUES('C', 4, 7, 1);
INSERT INTO Greatests VALUES('D', 3, 3, 8);

注意这里的第一列改成了key1，如果是key会报错，可能是关键字或者是mysql版本等乱七八糟的原因

解答

SELECT key1,case when x >= y and x >=z THEN xwhen y >=x and y >= z THEN yELSE z end as greatests
FROMgreatests