Join in SQL statements)

The join keywords in SQL statements are commonly used and not easy to understand. The following example provides a simple explanation-creating tables Table1 and Table2:
Create Table Table1 (ID int, name varchar (10 ))
Create Table Table2 (ID int, score INT)
Insert into Table1 select 1, 'lil'
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
Such as table
-------------------------------------------------
Table1 | Table2 |
-------------------------------------------------
ID name | ID score |
1 Lee | 1 90 |
2 Zhang | 2 100 |
4 Wang | 3 70 |
-------------------------------------------------

Run the following commands in the query Analyzer:

1. External Connection
1. Concept: including left Outer Join, right Outer Join or complete external join

2. Left join: left join or left Outer Join
(1) The result set of the left Outer Join includes all rows in the left table specified in the left outer clause, not just the rows matched by the join column. If a row in the left table does not match a row in the right table, all the selection list columns in the right table in the row of the associated result set are null ).
(2) SQL statements
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
------------------------------
Note: all the clauses containing Table 1 return the corresponding fields of Table 2 based on the specified conditions. The non-conforming fields are displayed as null.

3. Right join: Right join or right Outer Join
(1) The right outer join is the reverse join of the left Outer Join. All rows in the right table are returned. If a row in the right table does not match a row in the left table, a null value is returned for the left table.
(2) SQL statements
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: all the clauses containing Table 2 return the corresponding fields of Table 1 Based on the specified conditions. The non-conforming fields are displayed as null.

4. Complete External join: Full join or full outer join
(1) The Complete External join returns all rows in the left and right tables. If a row does not match a row in another table, the selection list column of the other table contains a null value. If there are matched rows between tables, the entire result set row contains the data value of the base table.
(2) SQL statements
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 3 70
------------------------------
Note: returns the sum of left and right connections (see upper left and right connections)

2. Internal Connection
1. Concept: inner join is a join that uses a comparison operator to compare the values of the columns to be joined.

2. Inner join: Join or inner join

3. SQL statements
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 the Table1 and Table2 columns that meet the conditions are returned.

4. equivalent (same as the following execution)
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: cross join can only use where, not on)

Iii. Cross-join (complete)

1. Concept: A cross join without a where clause will generate the Cartesian product of the table involved in the join. The number of rows in the first table multiplied by the number of rows in the second table is equal to the size of the Cartesian result set. (Table1 and Table2 generate 3*3 = 9 records)

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

3. SQL statements
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: 3*3 = 9 records are returned, that is, Cartesian product.

4. equivalent (same as the following execution)
A: Select * From Table1, Table2