Mysql Alter Constraint
A constraint is a property in SQL assigned to a column or the set of columns in a table that prevents certain types of inconsistent data values from being placed in the column of the table.
Mysql Alter Table Identity
Mysql Alter Table Identity is used to change the existing structure of table and add ID column as identity key that is used to uniquely define the records in the table.
PHP SQL Left Join
JOIN is used to query data from two or more tables. The left join is used in case of need to return all rows from the left table, even if the right table doesn't have any match.