SQL: Left connection, right connection, inner connection

Example:   ----------------------   ---------------------------  a table     ID   name       b table   ID   Job   parent_id              1   Sheets 31     1              2   John Doe 2 2              3   Wang Wu             3    Relationship between a.ID and parent_id in 4--------------------------------------------------

Original table above

1. Left connection:

Official explanation: The LEFT join returns records that include all the records in the left table and the equivalent of the join fields in the right table

Left JOIN connect     Select   a.*,b.*   from   a   left   join   b     on   a.id=b.parent_id         result is       1   Sheet 3                   1     1     2   John Doe 2 2     3   Wang Wu                  null  

2. Right Connection:

Official explanation: Right join returns records that include all records in the right table Uhe and the join fields in the left table are equal

Right connect     Select   a.*,b.*   from   a   to   join   b     on   a.id=b.parent_id         The result is       1   sheets 3                   1     1     2   John Doe                  2 2     null 3     4   

3. Internal connection:

Official explanation: Inner join (equivalent connection) returns two tables this is a line that joins fields that are equal

Internal connection     Select   a.*,b.*   from   a   inner   join   b     on   a.id=b.parent_id         result is       1   Sheet 3                   1     1     2   John Doe                  2     2   

SQL: Left connection, right connection, inner connection