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 authors.city = publishers.city

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.

Join Operators
Joins that match columns on the basis of equality are called equijoins.
Datatypes in Join Columns
The columns being joined must have the same or compatible datatypes. Use the convert function when comparing columns that are assigned datatypes that cannot be implicitly converted. Columns being joined need not have the same name, although they often do.
Joins and Text and Image Columns
You cannot use joins for columns containing text or image values. You can, however, compare the lengths of text columns from two tables with a where clause.

Parent topic: How Joins are Structured

Related concepts

How Joins are Processed