How to use join in sql

Regarding the join keyword in sql statements, it is a commonly used keyword but not easy to understand. The following example gives a simple explanation - create tables table1, table2:
create table table1(id int,name varchar(10))
create table table2(id int,score int)
insert into table1 select 1,'lee'
insert into table1 select 2,'zhang'
insert into table1 select 4,'wang'
insert into table2 select 1,90
insert into table2 select 2,100
insert into table2 select 3,70
as in the table
---------------------- ------------------------
table1 | table2 |
------------------------ --------------------------------
id name |id score |
1 lee |1 90 |
2 zhang |2 100 |
4 wang |3 70 |
------------------------------------------------ -------

The following are all executed in the query analyzer

1. Outer join
1. Concept: including left outer join, right outer join or complete outer join

2. Left join: left join or left outer join
(1)The result set of a left outer join includes all rows of the left table specified in the LEFT OUTER clause, not just the rows matched by the join columns. If a row in the left table has no matching row in the right table, all select list columns of the right table will be null in the associated result set row.
(2)sql statement
select * from table1 left join table2 on table1.id=table2.id
-------------Result-------------
id name id score
---------------------------------
1 lee 1 90
2 zhang 2 100
4 wang NULL NULL
---------------------------------
Note: Contains all clauses of table1 and returns the corresponding fields of table2 according to the specified conditions , non-matching ones are displayed as null

3. Right join: right join or right outer join
(1) Right outer join is the reverse join of left outer join. All rows from the right table will be returned. If a row in the right table has no matching row in the left table, NULL will be returned for the left table.
(2)sql statement
select * from table1 right join table2 on table1.id=table2.id
-------------Result-------------
id name id score
---------------------------------
1 lee 1 90
2 zhang 2 100
NULL NULL 3 70
--------------------------------
Note: Contains all clauses of table2 and returns the corresponding fields of table1 according to the specified conditions. , non-matching ones are displayed as null

4. Full outer join: full join or full outer join
(1) Full outer join returns all rows in the left table and right table. When a row has no matching row in another table, the other table's select list column contains null values. If there are matching rows between tables, the entire result set row contains the data values ​​from the base table.
(2)sql statement
select * from table1 full join table2 on table1.id=table2.id
-------------Result-------------
id name id score
---------------------------------
1 lee 1 90
2 zhang 2 100
4 wang NULL NULL
NULL NULL 3 70
---------------------------------
Note: Returns the sum of left and right concatenations (see above left, Right join)

2. Inner join
1. Concept: Inner join is a join that uses comparison operators to compare the values ​​of the columns to be joined

2. Inner join: join or inner join

3.sql statement
select * from table1 join table2 on table1.id=table2.id
-------------Result-------------
id name id score
----- ----------------------------------
1 lee 1 90
2 zhang 2 100
-------------- ----------------
Note: Only return the columns of table1 and table2 that meet the conditions

4. Equivalent (same execution effect as the following)
A: select a.*, b.* from table1 a,table2 b where a.id=b.id
B:select * from table1 cross join table2 where table1.id=table2.id (Note: only where can be used to add conditions after cross join, not on)

3. Cross join (complete)

1. Concept: A cross join without a WHERE clause will produce a Cartesian product of the tables involved in the join. The number of rows in the first table multiplied by the number of rows in the second table equals the size of the Cartesian product result set. (The cross join of table1 and table2 produces 3*3=9 records)

2. Cross join: cross join (without condition where...)

3.sql statement
select * from table1 cross join table2
-- ----------Result-------------
id name id score
------------------ ----------
1 lee 1 90
2 zhang 1 90
4 wang 1 90
1 lee 2 100
2 zhang 2 100
4 wang 2 100
1 lee 3 70
2 zhang 3 70
4 wang 3 70
---------------------------------
Note: Return 3*3=9 records, that is Cartesian product

4. Equivalent (same execution effect as the following)
A:select * from table1,table2