[turn to] MySQL operation practice (III): table connection

MySQL Practical operation （ 3、 ... and ）： Table join

1. Simple connection

① inner Join

Internal connection is also called equivalent connection .ANSI SQL Specifications are preferred INNER JOIN grammar .
Inner join is divided into implicit inner join and explicit inner join , The two queries have the same effect , It's just that the grammar is different .

Implicit inner connection ：WHERE Clause
FROM tb1_name, tb2_name, tb3_name
WHERE condition_1 AND condition_2
Explicit inner join ：INNER JOIN grammar
FROM tb1_name
INNER JOIN tb2_name ON condition_1
INNER JOIN tb3_name ON condition_2

Application scenarios ： Count the students who took the exam and their grades in various subjects
# Implicit inner connection

mysql> SELECT a.stu_id, stu_name, a.lesson_id, lesson_name, score    -> FROM t_score a, t_stu_profile b, t_lesson c    -> WHERE a.stu_id = b.stu_id AND a.lesson_id = c.lesson_id    -> ORDER BY stu_id, lesson_id    -> ;+--------+----------+-----------+-------------+-------+| stu_id | stu_name | lesson_id | lesson_name | score |+--------+----------+-----------+-------------+-------+|      1 |  Guo Dong      | L001      |  Chinese language and literature         |    90 ||      1 |  Guo Dong      | L002      |  data         |    86 ||      2 |  Lixi      | L001      |  Chinese language and literature         |    84 ||      2 |  Lixi      | L002      |  data         |    90 ||      2 |  Lixi      | L003      |  English         |    86 ||      2 |  Lixi      | L004      |  Physics         |    75 ||      2 |  Lixi      | L005      |  chemical         |    77 ||      3 |  Zhang Bei      | L001      |  Chinese language and literature         |   100 ||      3 |  Zhang Bei      | L002      |  data         |    91 ||      3 |  Zhang Bei      | L003      |  English         |    85 ||      4 |  Qian Nan      | L001      |  Chinese language and literature         |    99 ||      4 |  Qian Nan      | L002      |  data         |    88 ||      4 |  Qian Nan      | L003      |  English         |    66 ||      4 |  Qian Nan      | L005      |  chemical         |    98 |+--------+----------+-----------+-------------+-------+

# Explicit inner join

mysql> SELECT a.stu_id, stu_name, a.lesson_id, lesson_name, score    -> FROM t_score a    -> INNER JOIN t_stu_profile b    -> ON a.stu_id = b.stu_id    -> INNER JOIN t_lesson c    -> ON a.lesson_id = c.lesson_id    -> ORDER BY stu_id, lesson_id    -> ;

2. Advanced connection

① Self coupling

Self join is usually used as an external statement instead of a subquery statement used to retrieve data from the same table . Although the final result is the same , But sometimes processing joins is much faster than processing subqueries .

Application scenarios ： I found a 100 There is a problem with the score , Need to inquire 100 Is there any problem with the grades of other students in this course
Subquery ： First find 100 The name of this course （lesson_id）, Then find out the grades of other students in this course .

mysql> SELECT stu_id, score    -> FROM t_score    -> WHERE lesson_id = (    ->     SELECT lesson_id    ->     FROM t_score    ->     WHERE score = 100)    -> ;+--------+-------+| stu_id | score |+--------+-------+|      1 |    98 ||      2 |    84 ||      3 |   100 ||      4 |    99 |+--------+-------+

Self coupling ： The two tables needed in this query are actually the same table , therefore t_score Table in FROM The clause appears twice . Although it's legal , But yes t_score Table references are ambiguous , because MySQL I don't know the quotation is t_score Which instance of the table .

mysql> SELECT a.stu_id, a.score    -> FROM t_score a, t_score b    -> WHERE a.lesson_id = b.lesson_id AND b.score = 100    -> ;+--------+-------+| stu_id | score |+--------+-------+|      1 |    98 ||      2 |    84 ||      3 |   100 ||      4 |    99 |+--------+-------+

② natural join

Whenever tables are joined , There should be at least one column in more than one table （ Joined columns ）. The standard join returns all data , Even the same column appears multiple times .
Natural connections exclude multiple occurrences , Make each column return only once .

wildcard * Only for watches t_stu_profile Use , All other columns are clearly listed , So no duplicate columns are retrieved .

mysql> SELECT b.*, a.lesson_id, c.lesson_name, a.score    -> FROM t_score a, t_stu_profile b, t_lesson c    -> WHERE a.stu_id = b.stu_id AND a.lesson_id = c.lesson_id    -> ;

in fact , Every internal connection established so far is a natural connection , It is likely that internal connections that are not natural connections will never be used .

③ outer join

Many joins associate rows in one table with rows in another , But sometimes you need to include rows that have no associated rows . for example , You need to count how many orders each customer has placed , Including customers who have not yet placed orders .
that , A join contains rows that have no associated rows in the related table , This type of connection is called an external connection .

Application scenarios ： Count the grades of each student , Including students who did not take the exam
outer join ： The external connection is divided into left external connection （LEFT OUTER JOIN） And right outer connection （RIGHT OUTER JOIN）

mysql> SELECT a.stu_id, stu_name, lesson_id, score    -> FROM t_score a    -> RIGHT OUTER JOIN t_stu_profile b    -> ON a.stu_id = b.stu_id    -> ;+--------+----------+-----------+-------+| stu_id | stu_name | lesson_id | score |+--------+----------+-----------+-------+|      1 |  Guo Dong      | L001      |    98 ||      1 |  Guo Dong      | L002      |    86 ||      1 |  Guo Dong      | L003      |    79 ||      1 |  Guo Dong      | L004      |    88 ||      1 |  Guo Dong      | L005      |    98 ||      2 |  Lixi      | L001      |    84 ||      2 |  Lixi      | L002      |    90 ||      2 |  Lixi      | L003      |    86 ||      2 |  Lixi      | L004      |    75 ||      2 |  Lixi      | L005      |    77 ||      3 |  Zhang Bei      | L001      |   100 ||      3 |  Zhang Bei      | L002      |    91 ||      3 |  Zhang Bei      | L003      |    85 ||      3 |  Zhang Bei      | L004      |    79 ||      3 |  Zhang Bei      | L005      |    85 ||      4 |  Qian Nan      | L001      |    99 ||      4 |  Qian Nan      | L002      |    88 ||      4 |  Qian Nan      | L003      |    66 ||      4 |  Qian Nan      | L004      |    66 ||      4 |  Qian Nan      | L005      |    98 ||   NULL |  Wang Wu      | NULL      |  NULL ||   NULL |  Zhao Qi      | NULL      |  NULL |+--------+----------+-----------+-------+

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