where条件子句
作用:检索数据中符合条件的值
搜索的条件由一个或者多个表达式组成!结果 布尔值表示
逻辑运算符
-- ------------------------------------------------- -------------------where------------------------- 查询大一和大二的 SELECT `gradename` FROM gradeWHERE gradename ==大一 AND gradename ==大二-- 模糊查询SELECT `gradename` FROM gradeWHERE gradename BETWEEN 大一 AND 大二 -- 除了5号之外的学生SELECT `gradeid` FROM gradeWHERE gradeid!=5-- 除了6号之外的学生SELECT `gradeid` FROM gradeWHERE NOT gradeid=6
模糊查询:比较运算符
-- ------------------------------------------ -------------------模糊查询--------------- 查询大一的同学-- Like结合%(代表0到任意字符) (一个字符)SELECT `gradename` FROM gradeWHERE gradename LIKE '大一%'-- 查询姓刘的同学-- Like结合%(代表0到任意字符) (_一个字符)SELECT `gradename` FROM gradeWHERE gradename LIKE '刘%'-- 查询姓刘的同学,而且姓后面只有一个字的SELECT `gradename` FROM gradeWHERE gradename LIKE '刘_'-- 查询姓刘的同学,而且姓后面只有两个字的SELECT `gradename` FROM gradeWHERE gradename LIKE '刘__'-- 查询名字中间有嘉字的同学 %嘉%SELECT `gradename` FROM gradeWHERE gradename LIKE '%嘉%'--关于 in (使用具体的值,或者对个值)-- 查询1,2,3号学生信息SELECT `gradeid`,`gradename` FROM gradeWHERE gradeid IN(1,2,3)-- 查询在北京的学生SELECT `gradeid`,`gradename` FROM gradeWHERE `address` IN ('北京','安徽')-- -----null / not null-------- 查询年级为空的学生 null ''SELECT `gradeid`,`gradename` FROM gradeWHERE gradename='' OR gradename IS NULL-- 查询不为空的同学SELECT `gradeid`,`gradename` FROM gradeWHERE `gradename` IS NOT NULL