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一、添加数据。
— 1.创建表，并插入一定的数据。
CREATE TABLE STUDENT(
ID INT,
USERNAME VARCHAR(20),
SERVLET INT,
JSP INT,
ADDRESS VARCHAR(30)
);
INSERT INTO STUDENT VALUES (1,'张三',23,84,'京口区');
INSERT INTO STUDENT VALUES (2,'李四',36,74,'润州区');
INSERT INTO STUDENT VALUES (3,'王五',74,24,'京口区');
INSERT INTO STUDENT VALUES (4,'赵六',87,85,'京口区');
INSERT INTO STUDENT VALUES (5,'李七',63,98,'大港区');
INSERT INTO STUDENT VALUES (6,'郑八',99,89,null);
INSERT INTO STUDENT VALUES (7,'孙九',60,84,'');
— 2.插入部分数据
INSERT INTO STUDENT (ID,USERNAME) VALUES(8,'李十十');
二、修改数据
–1.根据条件修改数据
UPDATE STUDENT SET SERVLET = 86 WHERE ID =1;
–2.修改多个字段的内容
UPDATE STUDENT SET SERVLET = 65,JSP = 73 WHERE ID = 2;
三、删除数据（暂无演示）
— 7.删除所有数据
— 可以带条件删除，只能删除表的数据，不能删除表的约束，即自增长数值接着上次删除的数值增长，删除数据可以回滚。
DELETE FROM STUDENT;
— 8.带条件删除
DELETE FROM STUDENT WHERE SID = 2;
— 9.另一种删除方式,输出表中所有数据
— 不能带条件删除，既可以删除表的数据，也可以删除表的约束，永久删除。
TRUNCATE TABLE STUDENT;
四、单表查询
— 1.查询所有列
SELECT * FROM STUDENT;
— 2.查询指定列
SELECT ID ,USERNAME FROM STUDENT;
— 3.查询时指定别名
— 在多表查询时经常使用表的别名
SELECT ID AS '编号' ,USERNAME AS '用户名' FROM STUDENT AS S;
— 4.A查询时添加常量列
SELECT ID ,USERNAME,'这是一个班' AS '备注' FROM STUDENT;
— 5.查询时合并列
— 查询每个学生的SERVLET和JSP的总成绩
— 合并列只能合并数值类型的字段。
SELECT ID ,USERNAME ,(SERVLET+JSP) AS '总成绩' FROM STUDENT;
— 6.查询去除重复记录
— 查询所有JSP成绩可以出现的情况
SELECT DISTINCT ADDRESS FROM STUDENT;
— 另一种语法
SELECT DISTINCT(ADDRESS) FROM STUDENT;
— 7.条件查询(WHERE)
— 7.1逻辑条件 AND OR
— 查询 SID为2，且姓名为李四的学生
SELECT * FROM STUDENT WHERE ID = 2 AND USERNAME = '李四';
— 7.1.1查询 SID为2，或姓名为张三的学生
SELECT * FROM STUDENT WHERE ID = 2 OR USERNAME = '张三';
— 7.2比较条件 > < >= <= == <> BETWEEN AND
— 7.2.1查询 SERVLET 成绩大于70分的学生
SELECT * FROM STUDENT WHERE SERVLET > 70;
— 7.2.2查询 SERVLET 成绩大于60分小于80分的学生
SELECT * FROM STUDENT WHERE SERVLET >60 AND SERVLET < 80;
— 7.2.3查询 SERVLET 成绩大于等于70小于等于80的学生
SELECT * FROM STUDENT WHERE SERVLET >=70 AND SERVLET <=80;
— 7.2.4另外一种语法
SELECT * FROM STUDENT WHERE SERVLET BETWEEN 70 AND 80;
— 7.2.5查询 姓名不等于张三的记录
SELECT * FROM STUDENT WHERE USERNAME <>'张三';
— 7.3判空(null 空字符串) IS NULL IS NOT NULL =” <>”
— 7.3.1判断NULL
SELECT * FROM STUDENT WHERE ADDRESS IS NULL;
— 7.3.2判断空字符串
SELECT * FROM STUDENT WHERE ADDRESS ='';
— 7.3.3查询地址为空的记录
SELECT * FROM STUDENT WHERE ADDRESS IS NULL OR ADDRESS = '';
— 7.3.4查询地址不为空的记录
SELECT * FROM STUDENT WHERE ADDRESS IS NOT NULL AND ADDRESS <>'';
— 7.4模糊条件 LIKE
— 通常使用以下替换的标记：%：表示任意字符;_：表示一个字符
— 7.4.1查询姓李学生的记录
SELECT * FROM STUDENT WHERE USERNAME LIKE '李%';
— 8.聚合查询
— 常用的聚合函数：SUM() AVG() MAX() MIN() COUNT()
— 8.1查询学生SERVLET 总成绩
SELECT SUM(SERVLET) AS 'SERVLET的总成绩' FROM STUDENT;
— 8.2查询 学生 SERVLET 的平均分
SELECT AVG(SERVLET) AS 'SERVLET的平均分' FROM STUDENT;
— 8.3查询当前 SERVLET 的最高分
SELECT MAX(SERVLET) AS '最高分' FROM STUDENT;
— 8.4查询 SERVLET的最低分
SELECT MIN(SERVLET) AS '最低分' FROM STUDENT;
— 查询当前有多少学生 COUNT(字段)
SELECT COUNT(*) FROM STUDENT;
— 9.分页查询 LIMIT 起始行，查询几行。主要用于分页。
— 查询第1,2条记录
SELECT * FROM STUDENT LIMIT 0,2;
— 10.排序，默认情况下按插入记录的顺序排序。ORDER BY
— 10.1ASC 正序 DESC 倒序
SELECT * FROM STUDENT ORDER BY ID DESC;
— 10.2按照SERVLET正序，再JSP倒序
SELECT * FROM STUDENT ORDER BY SERVLET ASC ,JSP DESC;
— 11.分组查询 GROUP BY
— 查询每个地区的人数
SELECT ADDRESS,COUNT(*) FROM STUDENT GROUP BY ADDRESS;
— 12.分组查询后筛选
— 查询分组后区域人数超过1个人的区域 GROUP BY 后面不能再跟条件where
SELECT ADDRESS ,COUNT(*) FROM STUDENT GROUP BY ADDRESS HAVING COUNT(*)>1;
