Left connection, inner connection

Left connection , It will save all the data of the table on the left , After combining with the right side according to the conditions . If the data on the left is not available on the right （ The primary key value on the right does not exist ）, The data in the right half of the connected table will be set to null.


Internal connection means that the common data of two tables will be preserved , Like two tables （id） The same will be summarized in a table .