The FROM clause is a mandatory clause in a SelectExpression. It
specifies the tables (TableExpression)
from which the other clauses of the query can access columns for use in expressions.
SyntaxFROM TableExpression [ , TableExpression ] *SELECT Cities.city_id
FROM Cities
WHERE city_id < 5
-- other types of TableExpressions
SELECT TABLENAME, ISINDEX
FROM SYS.SYSTABLES T, SYS.SYSCONGLOMERATES C
WHERE T.TABLEID = C.TABLEID
ORDER BY TABLENAME, ISINDEX;
-- force the join order
SELECT *
FROM Flights, FlightAvailability
WHERE FlightAvailability.flight_id = Flights.flight_id
AND FlightAvailability.segment_number = Flights.segment_number
AND Flights.flight_id < 'AA1115'
-- a TableExpression can be a joinOperation. Therefore
-- you can have multiple join operations in a FROM clause
SELECT COUNTRIES.COUNTRY, CITIES.CITY_NAME, FLIGHTS.DEST_AIRPORT
FROM COUNTRIES LEFT OUTER JOIN CITIES
ON COUNTRIES.COUNTRY_ISO_CODE = CITIES.COUNTRY_ISO_CODE
LEFT OUTER JOIN FLIGHTS
ON Cities.AIRPORT = FLIGHTS.DEST_AIRPORT;