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JOIN Predicate (join_predicate) Locate the document in its SAP Library structure

A JOIN predicate (join_predicate) specifies a JOIN. A JOIN predicate can be specified with one, two, or no OUTER JOIN indicators.

Syntax

<join_predicate> ::=
<expression> [<outer_join_indicator>] <comp_op> <expression> [<outer_join_indicator>]

<outer_join_indicator> ::= (+)

<comp_op> ::= < | > | <> | != | = | <= | >=
|
~= | ~< | ~> (for computers with ASCII code)

Explanation

Each expression must contain a column specification. A column specification must exist for the first and second expression so that both specifications refer to different table names or reference names.

Let x be the value of the first expression and y the value of the second expression. The values x and y must be comparable with one another.

The rules for comparison predicates apply here.

If at least one OUTER JOIN indicator outer_join_indicator is specified in a JOIN predicate of a search condition, the corresponding table expression must be based on exactly two tables, or the following must apply:

·        OUTER JOIN indicators are specified for only one of the tables in the FROM clause.

·        All of the JOIN predicates in this table to just one other table contain the OUTER JOIN indicator.

·        All other JOIN predicates contain no OUTER JOIN indicators.

If a JOIN requires more than two tables for the QUERY specification and if one of the rules above cannot be observed, a QUERY expression can also be used in the FROM clause.

Table rows are transferred to the result table only if they have a counterpart of the comparison operator comp_op in the other table specified in the JOIN predicate.

If each row in a table is to appear at least once in the result table, the OUTER JOIN indicator must be specified on the side of the comparison operator where the other table is specified.
If it is not possible to find at least one counterpart for a table row in the other table, a row is still created for that table row in the result table. The NULL value is then used for the output columns which are formed from the columns in the other table.
Since the OUTER JOIN indicator can be specified on both sides of the comparison operator if the table expression is based on just two tables, it can be ensured that each line in both tables appears at least once in the result table.

The JOIN predicate is a special case of the comparison predicate. The number of JOIN predicates in a search condition is limited to 128.

Example

JOIN predicate

SELECT reservation.rno, customer.name, customer.firstname, reservation.arrival, reservation.departure
  FROM customer, reservation
    WHERE customer.name = 'Porter' AND customer.cno = reservation.cno

Is there a reservation for the customer 'Porter'? If so, for what date?

RNO

NAME

FIRSTNAME

ARRIVAL

DEPARTURE

100

Porter

Jenny

2004-11-13

2004-11-15

110

Porter

Jenny

2004-12-24

2005-01-06

Specifying an OUTER JOIN indicator

SELECT hotel.hno, hotel.name, reservation.rno
  FROM hotel, reservation
    WHERE hotel.zip = '10019' AND hotel.hno = reservation.hno (+)

List all the hotels in the location with zip cope 10019 for which reservations exist as well as those for which a reservation does not exist. Missing reservation numbers are assigned a NULL value.

HNO

NAME

RNO

80

Midtown

100

80

Midtown

140

40

Eight Avenue

?

 

 

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