新增，更新a表的数据等于b表数据——sql

新增

让st_student中得id，st_name等于xs_student表得xh，xm

INSERT INTO 
	st_student (id,st_name) 
SELECT
	xh,xm
FROM
	xs_student
WHERE
	NOT EXISTS (
		SELECT
			*
		FROM
			st_student
		WHERE
			st_student.id = xs_student.xh
	);

//2019 更新成 2019级

update st_student set field_two=CONCAT(field_two,'级')

更新年级

UPDATE st_student,sys_dict_item
SET field_two = item_value
  WHERE
	  st_student.field_two = sys_dict_item.item_text