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  • mysql数据库外连接,内连接,自然连接

    create table join_teacher(
    id int primary key auto_increment,
    t_name varchar(10) not null,
    gender enum('male','female','secret') not null
    )engine innodb character set utf8;
    insert into join_teacher values
    (1,'韩信','male'),
    (2,'李白','male'),
    (3,'韩非子','secret');

    create table join_class(
    id int primary key auto_increment,
    c_name varchar(10) not null,
    root int not nclass values
    (1,'php0115',207),
    (2,'php0115',207),
    (3,'php0115',207);

    create table join_day(
    id int primary key auto_increment,
    t_id int not null,
    c_id int not null,
    days tinyint not null,
    begin_date date not null,
    end_date  date not null
    )engine innodb character set utf8;
    insert into join_day values
    (1,1,1,15,20130115,20130220),
    (2,2,2,20,20130222,20130325),
    (3,3,3,15,20130327,20130418);

    tbl_left inner join tbl_right on 连接条件
    inner join
    select join_teacher.t_name,join_teacher.gender,join_day.begin_date,join_day.end_date
    from join_teacher inner join join_day on
    join_teacher.id=join_day.t_id;

    select * from join_teacher inner join join_day on
    join_teacher.id=join_day.t_id;

    left outer join
    select * from join_teacher left outer join join_day on
    join_teacher.id=join_day.t_id;
    join 连接查询:将多个表的记录连接起来

    分类:内连接,外连接,自然连接
    内连接:数据内部的连接,要求:连接的多个数据必须存在才能连接
    外连接:如果负责连接的一个或多个数据不真实存在,则称之为外连接


    select s.*,si.* from info_class as c left join info_student as s on c.id=s.class_id
    left join info_student_info as si on s.id=si.id where c.class_name='php0331';

    select child.* from hd_cate as parent left join hd_cate as child on child.pid=parent.id where parent.name='MySQL';

    格式:
    select * from table1
    left join table2 on 条件
    left join table3 on 条件
    ......
    按列往后加信息ull
    )engine innodb character set utf8;
    insert into join_

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  • 原文地址:https://www.cnblogs.com/hhfhmf/p/4827582.html
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