Usage of JOIN in MySQL

JOIN is used for join table query, there are 5 main usages.The five usages are demonstrated below.

Create 2 tables at will, the structure is as follows

The field col1 is used to make the two tables have a field with the same name (but it is actually useless, because the query conditions need to be specified with ON, here is just to explain that if there is the same field name, it does not matter)

I. Cartesian product: CROSS JOIN

CROSS JOINMake all fields of the two tables directly perform Cartesian product, assuming there are m pieces of data in table 1 and n pieces of data in table 2, the number of results is m*n piecesp>

SELECT * FROM tab1 CROSS JOIN tab2

Results

Second, inner join: INNER JOIN

The inner join needs to use ON to specify the fields to be compared between the two tables, and the final result only displays the data that meets the conditions

SELECT * FROM tab1 INNER JOIN tab2 ON tab1.id1 = tab2.id2

Results

Note that Inner joins only connect data that satisfy the ON condition, which is different from Cartesian product

Third, left join: LEFTJOIN

Left join can be regarded as the basis of inner join, and the data in the left table that does not meet the ON condition is also displayed, but the data in the right table part of the result is NULL

SELECT * FROM tab1 LEFT JOIN tab2 ON tab1.id1 = tab2.id2

Results

Fourth, right join: RIGHTJOIN

Right join is the exact opposite of left join

SELECT * FROM tab1 RIGHT JOIN tab2 ON tab1.id1 = tab2.id2

Results

5. Full connection: OUTERJOIN

Full join is the union of left join and right join, but MySQL does not support full join writing

SELECT * FROM tab1 OUTER JOIN tab2 ON tab1.id1 = tab2.id2

But you can use UNION to combine the result of left join and right join instead

SELECT * FROM tab1 LEFT JOIN tab2 ON tab1.id1 = tab2.id2UNIONSELECT * FROM tab1 RIGHT JOIN tab2 ON tab1.id1 = tab2.id2

Results

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