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Don Burleson Blog

SQL equality joins

Equality Joins

In an equality join, the tables are related using the equals (=) sign. This is the most common join and was used in the example at the beginning of this section. Basically, an equal join will compare the related values and return only those joined rows where they are the same.

SQL> select

2 author_last_name,

3 book_title

4 from

5 author a join book_author ba on (a.author_key = ba.author_key)

6 join book b on (ba.book_key = b.book_key);

AUTHOR_LAST_NAME BOOK_TITLE

----------------------------- --------------------------

hester windows success

jones windows success

jeckle piano greats

weaton piano greats

withers DOS for dummies

shagger The zen of auto repair

smith zero loss finance

jeckle operations research theory

shagger non violins in the workplace

weaton UNIX for experts

hester pay no taxes and go to jail

jeckle the fall of microsoft

shagger writers market

smith managing stress

shagger bears are people too

weaton reduce spending the republican way

petty the willow weeps no more

hester oracle9i sql tuning

shagger was george washington feeble?

jones was george washington feeble?

jones cooking light

jones never eat meat

withers never eat meat

weaton never eat meat

shagger how to housebreak your horse

25 rows selected.

Remember that a book can have more than one author.

One ANSI join that is a type of equality join is the natural join. A natural join is where tables are joined by matching column names and column values. As stated earlier, I do not like this join because it allows the database to determine the matching columns. To me, it is ambiguous to the code reader. Here is the query above rewritten using a natural join.

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