MySQL基础函数

/*NULL=======================================*/
SELECT IFNULL(NULL,'IS NULL');
#IS NULL

/*date type==================================*/
SELECT DATE_FORMAT(NOW(),'%Y-%m-%d %H:%i:%s');
#2021-04-02 10:13:52

SELECT STR_TO_DATE('2021-04-02','%Y-%m-%d');
#2021-04-02

SELECT UNIX_TIMESTAMP(NOW());
#1617329776
SELECT UNIX_TIMESTAMP('2021-04-02');
#1617292800

SELECT FROM_UNIXTIME(1617329776);
#2021-04-02 10:16:16
SELECT FROM_UNIXTIME(1617329776,'%Y-%m-%d %H:%i:%s');
#2021-04-02 10:16:16

SELECT DATE_ADD('2021-04-02',INTERVAL 1 day);
#2021-04-03
SELECT DATE_ADD(NOW(),INTERVAL 1 day);
#2021-04-03 10:17:57

SELECT DATEDIFF(NOW(),'2021-03-01');
#32
SELECT DATEDIFF('2021-04-02','2021-04-01');
#1

/*char type==================================*/
SELECT CONCAT('hello','-',123);
#hello-123