When you join two or more tables, the columns being compared must have similar values—that is, values using the same or similar datatypes.
There are several types of joins, such as equijoins, natural joins, and outer joins. The most common join, the equijoin, is based on equality. The following join finds the names of authors and publishers located in the same city:
select au_fname, au_lname, pub_name from authors, publishers where authors.city = publishers.city
au_fname au_lname pub_name -------- -------- -------------------- Cheryl Carson Algodata Infosystems Abraham Bennet Algodata Infosystems (2 rows affected)
Because the query draws on information contained in two separate tables, publishers and authors, you need a join to retrieve the requested information. This statement joins the publishers and authors tables using the city column as the link:
where authors.city = publishers.city
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