A JOIN command is queries that combine data from more than 1 table.
For two tables that want to join together need to have some data in common, like unique id that link this 2 tables together.
Normal join will need a joining condition or connecting column to display out the joined data. 1 joining condition needs when we want to join 2 tables. If more than 2 tables want to join together, more joining condition or connecting column needed.
A connecting column should have values that match easily for both tables. Connecting columns almost always have the same datatype. The value in the connecting columns are join compatible or can say that the value are come from the same general class of data.
SQL JOIN syntax:
SELECT [COLUMN 1], [COLUMN 2]
FROM [TABLE 1] , [TABLE 2]
WHERE [TABLE 1].[COLUMN NAME 1] JOIN OPERATOR [TABLE 2].[COLUMN NAME 2]
EXAMPLE :
Let’s say, we only want to join 2 tables below and display only PlayerName and DepartmentName
Table 1: GameScores
| PlayerName | DepartmentId | Scores |
| Jason | 1 | 3000 |
| Irene | 1 | 1500 |
| Jane | 2 | 1000 |
| David | 2 | 2500 |
| Paul | 3 | 2000 |
| James | 3 | 2000 |
Table 2: Departments
| DepartmentId | DepartmentName |
| 1 | IT |
| 2 | Marketing |
| 3 | HR |
The joining Condition will be DepartmentId of both tables.
SQL statement :
SELECT PlayerName, DepartmentName
FROM GameScores a, Departments b
WHERE a.DepartmentId = b.DepartmentId
Result:
| PlayerName | DepartmentName |
| Jason | IT |
| Irene | IT |
| Jane | Marketing |
| David | Marketing |
| Paul | HR |
| James | HR |