The where Clause

Use the where clause to determine which rows are included in the results of a join statement.

where specifies the connection between the tables and views named in the from clause. Qualify column names if there is ambiguity about the table or view to which they belong. For example:
where = 

This where clause gives the names of the columns to be joined, qualified by table names if necessary, and the join operator—often equality, sometimes “greater than” or “less than.”

Note: You will get unexpected results if you omit the where clause of a join. Without a where clause, any of the join queries discussed so far produces 69 rows instead of 2.

The where clause of a join statement can include conditions other than the one that links columns from different tables. In other words, you can include a join operation and a select operation in the same SQL statement.

Related concepts
How Joins are Processed