FROM Clause

The FROM clause is used to construct a result table from a database table or from a join of database tables.

Syntax Syntax

<from clause> ::= FROM <table specification>
                             ( ',' <table specification> )*.

<table specification> ::= <table reference>
                                       | <joined table>.

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The result of a <from clause> is the extended Cartesian product (ECP) of the <table specification>s in the <from clause>. The ECP of two <table specification>s (A, B) is constructed as follows:

Every row from A is concatenated with every row from B. The cardinality of ECP is the product of the cardinalities of A and B, the degree of ECP is the sum of the degrees of A and B.

Table A:
column1	column2
1	a
2	a

Table B:
column1	column2
1	b
2	b

ECP:
A.column1	A.column2	B.column1	B.column2
1	A	1	B
1	A	2	B
2	A	1	B
2	A	2	B

If only one <table specification> is specified the result of the <from clause> is this <table specification>.

Example Example

SELECT employee_name, manager_name 
    FROM employees AS e, managers AS m 
    WHERE e.manager_id = m.manager_id

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The FROM Clause. This query produces a list of all employees that have a manager along with their respective managers. Here, the FROM clause contains a list of two table references, each having a table alias.