Left connection and right connection of MySQL (the difference between inner connection and natural connection)

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obtain boy 、girl Table download address

One 、 Internal connection query inner join

keyword ：inner join on

sentence ：select * from a_table a inner join b_table b on a.a_id = b.b_id;

explain ： Combine records from two tables , Return the records matching the associated fields , That is, return the intersection of two tables （ shadow ） part .

Case explanation ： stay boy Table and girl There are two tables in the table hid Field consistent name （gname,bname）,boy Table and girl The table is as follows ：

Internal connection query method is adopted ：

SELECT boy.hid,boy.bname,girl.gname FROM boy INNER JOIN girl ON girl.hid = boy.hid;

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The query results are as follows ：

Two 、 Left connection query left join

keyword ：left join on / left outer join on

sentence ：SELECT * FROM a_table a left join b_table b ON a.a_id = b.b_id;

explain ： left join yes left outer join Abbreviation , Its full name is left outer join , It's one of the outer connections . Left ( Outside ) Connect , The left table (a_table) All of the records will be represented , The right table (b_table) Only records that match the search criteria are displayed . The missing records in the right table are NULL.

Case explanation ： stay boy Table and girl Left connection query in the table ,boy Table and girl The table is as follows ：

Internal connection query method is adopted ：

SELECT boy.hid,boy.bname,girl.gname FROM boy LEFT JOIN girl ON girl.hid = boy.hid;

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The query results are as follows ：

3、 ... and 、 The right connection right join

keyword ：right join on / right outer join on

sentence ：SELECT * FROM a_table a right outer join b_table b on a.a_id = b.b_id;

explain ：right join yes right outer join Abbreviation , Its full name is right outer connection , It's one of the outer connections . And left ( Outside ) The connection is the opposite , Right ( Outside ) Connect , The left table (a_table) Only records that match the search criteria are displayed , The right table (b_table) All of the records will be represented . All the places that are not recorded in the left table are NULL.

Case explanation ： stay boy Table and girl Right link query in the table ,boy Table and girl The table is as follows ：

Internal connection query method is adopted ：

SELECT boy.hid,boy.bname,girl.gname FROM boy RIGHT JOIN girl ON girl.hid = boy.hid;

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The query results are as follows ：

Four 、 Full connection union

keyword ：union /union all

sentence ：(select colum1,colum2…columN from tableA ) union (select colum1,colum2…columN from tableB )

or (select colum1,colum2…columN from tableA ) union all (select colum1,colum2…columN from tableB );

union Statement notes ：

1. adopt union Connected SQL They must have the same number of columns taken out separately ;

2. Do not require the merged table column names to be the same , In the first sql The names listed in the table shall prevail ;

3. Use union when , Exactly equal lines , Will be merged , Because merging is time consuming , Generally, it is not used directly union A merger , It's usually used union all A merger ;

4. By union Connected sql Clause , Don't write in a single sentence order by , Because there will be no sorting effect . But you can sort the final result set ;

(select id,name from A order by id) union all (select id,name from B order by id); // No sorting effect

(select id,name from A ) union all (select id,name from B ) order by id; // It has sorting effect

Case explanation ： take a Table and b Table merge , The table structure is as follows ：

use union Full connection ：

union It will automatically remove the completely repeated data ,a、b In the table ”c” The values are 15, So just one line .

use union all Full connection ：

union all Will keep those duplicate data ;

Left and right connecting exercises ：

Write... As required according to the given table structure SQL sentence .t surface ( namely Team surface ) and m surface ( namely Match surface ) The structure is as follows ：

t surface ( namely Team surface ) and m surface ( namely Match surface ) Is as follows ：

t Table and m Table download address

m surface ( namely Match surface ) Of hostTeamID And guestTeamID Both with t surface ( namely Team surface ) Medium teamID relation . Please find out 2006-6-1 To 2006-7-1 All the competitions between , And list... In the following form ： Bayern Munich 2：0 Bremen 2006-6-21

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Solution ：

First step ： Start with m Table left connection t surface , Find out m In the table hid This column corresponds to the competition information :

SELECT m.mid,t.tname,m.mres,m.matime as time FROM m LEFT JOIN t ON t.tid = m.hid;

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Query results are recorded as result sets t1 ,t1 The table is as follows ：

The second step ： Start with m Table left connection t surface , Find out m In the table gid This column corresponds to the competition information :

SELECT m.mid,t.tname,m.mres,m.matime FROM m LEFT JOIN t ON t.tid = m.gid;

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Query results are recorded as result sets t2 ,t2 The table is as follows ：

The third step ： As a result set t1 Left join query result set based on t2, The query condition is the match number of both （mid） identical .

SELECT t1.tname,t1.mres,t2.tname,t1.time FROM 
(SELECT m.mid,t.tname,m.mres,m.matime as time FROM m LEFT JOIN t ON t.tid = m.hid) 
as t1 
LEFT JOIN 
(SELECT m.mid,t.tname,m.mres,m.matime as time FROM m LEFT JOIN t ON t.tid = m.gid) 
as t2 
ON t1.mid = t2.mid WHERE t1.time BETWEEN '2006-06-01' AND '2006-07-01';

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The query results are as follows ：

Full connection exercises ：

A Table and B The table structure is as follows , Please merge the two tables ：

Merger requirements ：A In the table a:5,B In the table a：5, Therefore, in the consolidated table a The corresponding value is 10; The samples of results required are as follows ：

use union all Full connection , And then use from Subquery ：

SELECT id,SUM(num) as num FROM ((SELECT id,num FROM a) UNION ALL(SELECT id,num FROM b)) as tb GROUP BY id;

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