SELECT * FROM tbl_name;
SELECT tbl_name.* FROM tbl_name;
查询指定字段
SELECT test1,test2 FROM tbl_name;
可以通过db_name.tbl_name查询哪个数据库的哪张表
例SELECT * FROM test.user; SELECT 表名.字段名 FROM 数据库名.表名;
SELECT test1.id,test2.username FROM project.user;
给表起别名
SELECT 字段名 FROM tbl_name AS a; 现在a是别名了，查询某表的字段时可以用a代替了
SELECT a.字段名1,a.字段名2 FROM tbl_name AS a;
给字段起别名
SELECT id AS '编号',username AS '用户名',age AS '年龄 FROM user；
带WHERE 条件查询
运算符=,<,>,<>,<=>等等，<=>这符号为null而设
SELECT * FROM tbl_name WHERE test1=1;
SELECT * FROM tbl_name WHERE test1=null; 这写错错误，要想查询条件是test1=null的内容时，要这样写
SELECT * FROM tbl_name WHERE test1<=>null; 或者SELECT * FROM tbl_name WHERE test1 is null;
BETWEEN AND查询 几到几之间的记录
SELECT * FROM tbl_name WHERE id BETWEEN 5 AND 10; 查询id5至10的记录;
IN 指定集合
SELECT * FROM tbl_name WHERE id IN(1,3,5,,7,100); 查询id 为1,3,5,7,100的记录
SELECT * FROM tbl_name WHERE username IN('张三','李四','丁五'); 查询username为张三，李四，丁五的记录
模糊查询 LIKE
%表示0个或1个或多个任意字符
_表示1个任意字符
SELECT * FROM tbl_name WHERE username LIKE '张%'; 查询姓张的用户；
SELECT * FROM tbl_name WHERE username LIKE '%in%'; 查询包含in的用户;
SELECT * FROM tbl_name WHERE username LIKE '___'; 查询用户名为3位数的用户;
关键字AND 代表并且
SELECT * FROM tbl_name WHERE username='niuB' AND age=18;
SELECT * FROM tbl_name WHERE username='niuB' AND age=18 AND address='北京;
关键字GROUP BY 分组查询 注：用分组查询只会显示该组的第一条记录
SELECT * FROM tbl_name GROUP BY sex; 以性别为分组查询
