#select sname as "姓名", length(sname) as "姓名的字数" from stu;
#select sid from stu where age>20;
SELECT CONCAT(‘hello‘,‘world‘);
SELECT CONCAT_WS(‘-‘,‘hello‘,‘world‘);

SELECT LEFT(‘right‘,3);

SELECT TRIM(‘     Hello   world     ‘);

SELECT SUBSTRING(‘hello‘,2);

SELECT CEIL(2.4), FLOOR(2.2),ROUND(2.6);
SELECT ROUND(2.123673,4),TRUNCATE(2.2342341,4);

SELECT NOW()
SELECT SYSDATE()

SELECT CURRENT_DATE();
SELECT CURRENT_DATE();
SELECT CURRENT_TIMESTAMP();