MySQL 数据库查询练习：单表查询操作

a) SELECT * FROM emp WHERE deptno = 20; \nb) SELECT * FROM emp WHERE comm > sal; \nc) SELECT * FROM emp WHERE comm > sal * 0.2; \nd) SELECT * FROM emp WHERE (deptno = 10 AND job = "MANAGER") OR (deptno = 20 AND job = "CLERK"); \ne) SELECT * FROM emp WHERE job NOT IN ("MANAGER", "CLERK") AND sal >= 2000; \nf) SELECT * FROM emp WHERE comm IS NULL OR comm < 100; \ng) SELECT * FROM emp WHERE TIMESTAMPDIFF(YEAR, hiredate, CURDATE()) >= 10; \nh) SELECT CONCAT(UPPER(LEFT(ename, 1)), SUBSTRING(ename, 2)) AS ename FROM emp; \ni) SELECT * FROM emp WHERE MONTH(hiredate) = 2; \nj) SELECT ename, YEAR(hiredate) AS hire_year, MONTH(hiredate) AS hire_month FROM emp ORDER BY MONTH(hiredate), YEAR(hiredate);