Figure out the join keywords of MySQL (7 categories, clear at a glance)

Homo sapiens 2021-01-22 18:08:04
figure join keywords mysql categories


SQL Join Join clauses are used to combine records in two or more tables in the database .JOIN Is connected by using a combination of values common to two table fields .MySQL Only three of them are officially available join The way , Internal connection 、 Left connection and right connection , Other connection keywords are not supported . But we can achieve other connection effects through certain syntax .

1. Internal connection inner join

The most frequently used and important join is INNER JOIN. It's also called a Equivalent connection .

INNER JOIN By combining two tables based on join predicates ( surface 1 And table 2) Column values create a new result table . Query comparison table 1 Each row and table in 2 Each line in , Find all the rows that satisfy the join predicate . When the join predicate is satisfied , For each matching pair A and B The column values of are combined into a result row . Basic grammar :

SELECT tableA.column1, tableB.column2… FROM tableA INNER JOIN tableB ON tableA.common_field = tableB.common_field;

Sketch Map :

2. The left outer join left join

SQL LEFT JOIN Returns the left table of all rows , Even if there is no match in the left table . It means , If in ON Clause matches the table on the right 0( zero ) A record , Then the connection will still return one line in the result , But with NULL In each column in the right table . It means , A left join returns all values from the left table , Plus, if the right table doesn't match, use NULL Value instead of returning . Basic grammar :

SELECT tableA.column1, tableB.column2… FROM tableA LEFT JOIN tableB ON tableA.common_field = tableB.common_field;

Sketch Map :

3. Right connection right join

SQL RIGHT JOIN Return all the rows in the right table , Even if there is no match in the left table . It means , If ON Clause matches left table 0( zero ) Record , The connection still returns one row of results , But with NULL Instead of each column in the left table . It means , A right join returns all the values in the right table , Plus, if there is no matching left table, use NULL value .

Basic grammar : SELECT tableA.column1, tableB.column2… FROM tableA RIGHT JOIN tableB ON tableA.common_field = tableB.common_field;

Sketch Map :

4. Left connection

Start with this one ,MySQL Do not provide a formal one-step keyword , The effects are all simulated by ourselves . The left link is to get A Remove from table B The rest of the table content , That is to say A A unique part of the table . It can be seen as a result of removing the common parts of both sides in the left outer connection .

Basic grammar :

SELECT tableA.column1, tableA.column2… FROM tableA LEFT JOIN tableB ON tableA.common_field = tableB.common_field where tableB.column1 is null limit 10;

Sketch Map :

5. The right connection

Empathy , The right join is obtained from the result of the right outer join B The unique part of the watch

Basic grammar :

SELECT tableB.column1, tableB.column2… FROM tableA RIGHT JOIN tableB ON tableA.common_field = tableB.common_field; where tableA.column1 is null limit 10;

Sketch Map :

6. Full connection

Full connection, as the name suggests, is to get AB All the data in the two tables ,oracle Provides full join Keywords do this , however MySQL No, . however MySQL We can make use of union To achieve this effect ,union The role of is Merge the results of two queries .

Sketch Map :

7. Difference set

Remove the overlapped part from the full connection of two tables , That is, the collection of the specific parts of the two tables .

summary :

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