廢話不多說，直接進入正題

#數據準備

班級表class:

CREATE TABLE `class` (
  `class_no` int(2) unsigned zerofill NOT NULL AUTO_INCREMENT COMMENT ‘班級編號‘,
  `class_name` varchar(30) CHARACTER SET utf8 NOT NULL COMMENT ‘班級名稱‘,
  PRIMARY KEY (`class_no`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

insert into class values(1, ‘培優班
 
‘);
insert into class values(2, ‘普通班‘);
insert into class values(3, ‘提升班‘);

學生表student:

CREATE TABLE `student` (
  `stu_no` int(2) unsigned zerofill NOT NULL AUTO_INCREMENT COMMENT ‘學員編號‘,
  `stu_name` varchar(30) CHARACTER SET utf8 NOT NULL COMMENT ‘學員姓名‘,
  `stu_sex` varchar(3) CHARACTER SET utf8 NOT
 
 NULL COMMENT ‘學員性別‘,
  `stu_age` tinyint(2) unsigned zerofill DEFAULT NULL COMMENT ‘學員年代‘,
  `grade` double(5,2) unsigned zerofill DEFAULT NULL COMMENT ‘成績‘,
  `class_no` int(2) unsigned zerofill DEFAULT NULL COMMENT ‘所在班級編號‘,
  PRIMARY KEY (`stu_no`),
  KEY `class_no` (`class_no`)
) ENGINE=MyISAM AUTO_INCREMENT=
 
11 DEFAULT CHARSET=latin1;

insert into student values(01, ‘李白‘, ‘男‘, 18, 60, 01);
insert into student values(02, ‘杜甫‘, ‘男‘, 20, 76, 01);
insert into student values(03, ‘張飛‘, ‘男‘, 32, 80, 02);
insert into student values(04, ‘韓信‘, ‘男‘, 26, 98, 02);
insert into student values(05, ‘了龍‘, ‘男‘, 27, 56, 02);
insert into student values(06, ‘大喬‘, ‘女‘, 17, 88, 01);
insert into student values(07, ‘小喬‘, ‘女‘, 16, 96, 01);
insert into student values(08, ‘小喬‘, ‘女‘, 16, 90, 01);
insert into student values(09, ‘關哥‘, ‘男‘, 32, 80, 02);
insert into student values(10, ‘劉備‘, ‘男‘, 36, 98, null);

1: exists子查詢

如果子查詢有返回結果則為true,如果沒有返回值則為false

select * from student where exists(select * from student where grade = 80)

比如not exists：

select * from student where not exists(select * from student where grade = 80);

以上結果返回空,因為 not exists 返回了 false

select * from student where exists (select * from class where class.class_no = student.class_no);

上面的查詢可以看到，我們少了一條數據，第十條的clas_no 是null。。。所以這條數據是flase....而

class.class_no = student.class_no 為true的，就全部返回了

2: [union] 並合查詢

需求: 拿到01班級的最高成績 和 02班級的最低成績

我們一般會這樣

select max(grade) from student where class_no = 01;
select min(grade) from student where class_no = 02;

優化這個查詢我們可以這樣：

(select concat(‘1號班級最高成績:‘, max(grade)) ‘成績‘ from student where class_no = 01)
union
(select concat(‘2號班級最低成績:‘, min(grade)) ‘成績‘ from student where class_no = 02);

這裏再說下union 和union all的區別：

union:

(select class_no, stu_name, stu_age from student where class_no = 1)
union
(select class_no, stu_name, stu_age from student where class_no = 2);

union all:

(select class_no, stu_name, stu_age from student where class_no = 1)
union all
(select class_no, stu_name, stu_age from student where class_no = 2);

通過以上兩個查詢，我們可以看到：union並合查詢它會自動的去重復的記錄, 如果不想要去掉重復的記錄則可以使用 union all;

我們加個排序：

(select class_no, stu_name, stu_age from student where class_no = 1)
union all
(select class_no, stu_name, stu_age from student where class_no = 2) order by stu_age desc;

連接查詢的分類
1: 內連接
2: 外連接
3: 自然連接

1: inner join(內連接)
需求: 查詢出學員的學號, 姓名, 所在的班級名稱

select stu_no, stu_name, class_name from student  join class where `student`.class_no = `class`.class_no;

以上sql等同於：

select stu_no, stu_name, class_name from student join class where `student`.class_no = `class`.class_no;
select stu_no,stu_name,class_name from student,class where student.class_no = class.class_no;

內連接的inner字符可以不用寫

2: cross join(交叉連接,迪卡爾集) 沒有條件的內連接
例: select * from student cross join class;
例: select * from student inner join class;
例: select * from student cross join class where `student`.class_no = `class`.class_no;
ps: cross join 與 inner join 在使用上沒有區分，只是在mysql中把cross join定義成交叉連接而已

就寫到這把。。其他的連接方式也簡單，資料也很多啦。。。

mysql 關聯查詢技巧