Select all employee's name and bonus whose bonus is < 1000.
Table:Employee
+-------+--------+-----------+--------+ | empId | name | supervisor| salary | +-------+--------+-----------+--------+ | 1 | John | 3 | 1000 | | 2 | Dan | 3 | 2000 | | 3 | Brad | null | 4000 | | 4 | Thomas | 3 | 4000 | +-------+--------+-----------+--------+ empId is the primary key column for this table.
Table: Bonus
+-------+-------+ | empId | bonus | +-------+-------+ | 2 | 500 | | 4 | 2000 | +-------+-------+ empId is the primary key column for this table.
Example ouput:
+-------+-------+ | name | bonus | +-------+-------+ | John | null | | Dan | 500 | | Brad | null | +-------+-------+
OUTER JOIN and WHERE clause [Accepted]Intuition
Join table Employee with Bonus and then use WHERE clause to get the required records.
Algorithm
Since foreign key Bonus.empId refers to Employee.empId and some employees do not have bonus records, we can use OUTER JOIN to link these two tables as the first step.
SELECT Employee.name, Bonus.bonus FROM Employee LEFT OUTER JOIN Bonus ON Employee.empid = Bonus.empid ;
Note: "LEFT OUTER JOIN" could be written as "LEFT JOIN".
The output to run this code with the sample data is as below.
| name | bonus | |--------|-------| | Dan | 500 | | Thomas | 2000 | | Brad | | | John | |
The bonus value for 'Brad' and 'John' is empty, which is actually NULL in the database. "Conceptually, NULL means “a missing unknown value” and it is treated somewhat differently from other values." Check the Working with NULL Values in MySQL manual for more details. In addition, we have to use IS NULL or IS NOT NULL to compare a value with NULL.
At last, we can add a WHERE clause with the proper conditions to filter these records.
MySQL
SELECT Employee.name, Bonus.bonus FROM Employee LEFT JOIN Bonus ON Employee.empid = Bonus.empid WHERE bonus < 1000 OR bonus IS NULL ;