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 CommentsProcedural SQL (PSQL) Statements 
Implicit Joins

In the SQL:89 standard, the tables involved in a join were specified as a comma-delimited list in the FROM clause (in other words, a cross join).The join conditions were then specified in the WHERE clause among other search terms.This type of join is called an implicit join.

An example of an implicit join:

/*
 * A sample of all Detroit customers who
 * made a purchase.
 */
SELECT *
FROM customers c, sales s
WHERE s.cust_id = c.id AND c.city = 'Detroit'
Mixing Explicit and Implicit Joins

Mixing explicit and implicit joins is not recommend, but is allowed.However, some types of mixing are not supported by Firebird.

For example, the following query will raise the error “Column does not belong to referenced table”

SELECT *
FROM TA, TB
JOIN TC ON TA.COL1 = TC.COL1
WHERE TA.COL2 = TB.COL2

That is because the explicit join cannot see the TA table.However, the next query will complete without error, since the restriction is not violated.

SELECT *
FROM TA, TB
JOIN TC ON TB.COL1 = TC.COL1
WHERE TA.COL2 = TB.COL2