不等于:<>
判断为空的条件:null和空格(空字符串)
判断是否为null:xxxx is not null / xxxx is null
判断null:
SELECT * FROM student WHERE address IS NULL ;
判断空字符串:
SELECT * FROM student WHERE address='';
包括null和空字符串:
SELECT * FROM student WHERE address IS NULL OR address='';
模糊查询:like
SELECT * FROM score WHERE sname LIKE '大%';(%控制多个字符)
SELECT * FROM score WHERE sname LIKE '大_';(_控制一个字符)
SELECT * FROM score WHERE sname LIKE '__';(两位字符)
聚合查询:sum(求和)avg(平均数)max(最大值)min(最小值)count(总记录数)
SELECT SUM(js) FROM score WHERE sclass='j1018';
SELECT AVG(html) FROM score WHERE sclass='JAVA-1018';
SELECT MIN(html+js+jquery) FROM score;
SELECT COUNT(*) FROM score;(统计总共有做少数据:每一列都count一下,取最大值)
判空函数
IFNULL(html,0);
SELECT MIN(IFNULL(html,0)+js+jquery) FROM score;