A SELECT statement can retrieve data from two or more tables by performing a JOIN operation.
A join collects data from tables that have one specific column in common and combines the result into one result data set.
The join condition is specified in the WHERE clause, or with the ON keyword in ANSI syntax.
Table aliases like E and D keep the query short and readable.
What is an inner join
An inner join returns only the rows where both tables have matching values in the join columns.
Rows with no matching partner in the other table are dropped from the result.
It is the default join type: writing JOIN alone means INNER JOIN.
Example:-
-- Old style: join condition in the WHERE clause
SELECT E.LASTNAME, D.DEPTNAME
FROM EMPLOYEE E, DEPARTMENT D
WHERE E.WORKDEPT = D.DEPTNO;
-- ANSI style: INNER JOIN with ON
SELECT E.LASTNAME, D.DEPTNAME
FROM EMPLOYEE E INNER JOIN DEPARTMENT D
ON E.WORKDEPT = D.DEPTNO;
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