Database Connection and external connection (left connection, right connection, full connection)

Internal join: Check the data corresponding to the data in the two tables.
External join: Check the corresponding data based on a table (full join is based on multiple tables)

Student table
No name
1
2 B
3 C
4 d

Grade table
No grade
1 90
2 98
3 95

Join inner join (the data corresponding to the search condition is not listed in No4)
Syntax: Select * from student inner join grade on student. No = Grade. No
Result
Student. No name grade. No grade
1 A 1 90
2 B 2 98
3 C 3 95

Left join (all data in the left table, corresponding data in the right table)
Syntax: Select * from student left join grade on student. No = Grade. No
Result:
Student. No name grade. No grade
1 A 1 90
2 B 2 98
3 C 3 95
4 d

right join (all data in the right table, corresponding data in the left table)
Syntax: Select * from student right join grade on student. no = Grade. no
result:
student. no name grade. no Grade
1 A 1 90
2 B 2 98
3 C 3 95

Full connection
Syntax: Select * from student full join grade on student. No = Grade. No
Result:
No name grade
1 A 90
2 B 98
3 C 95
4 d
1 A 90
2 B 98
3 C 95

Note: you cannot directly use full join in access. You must use Union all to merge the left and right connections.