1、删除子表数据（联合查询删除）
delete t1
from RR.tblOeWkPublicOrderItem as t1
inner join RR.tblOeWkPublicOrderHeader t2 on t1.tran_code = t2.tran_code and t1.request_no = t2.request_no
where create_date < @min_date;
2、联合查询修改
UPDATE T1 t1
JOIN T2 t2 ON t1.id = t2.id AND t1.no = t2.no
SET t1.equipment_id = t2.equipment_id;
3、修改,1改成0，0改成1
UPDATE tableName
SET STATUS = CASE STATUS WHEN 0 THEN 1 WHEN 1 THEN 0 END WHERE id='1';
4、查询日期，七天前的年月日YYYY-MM-DD
SELECT DATE(DATE_SUB(NOW(), INTERVAL 7 DAY));
5、修改表数据集
ALTER TABLE tableA CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;
6、创建索引
CREATE INDEX idx_id ON tblUser (user_id);
CREATE INDEX idx_url ON tblUser (url(255));
7、
