mysql 關聯查詢技巧
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 NOTNULL 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