Gbase 8s extended external connection 1

The next outer link lists the customer's company name and all associated order numbers , If the customer has placed an order . If not

Yes , Still list the company name , And return... For the order number NULL value .

SELECT company, order_num FROM customer c, OUTER orders o

WHERE c.customer_num = o.customer_num;

Hierarchical query clause

Hierarchical clauses set conditions for recursive queries on table objects , In the rows of the table object , There is a hierarchy of parent-child dependencies . Including this sub

Syntactic SELECT Statements are called hierarchical queries .

Must be in SELECT Of the statement FROM Clause specifies the table object on which the hierarchical query operation is performed . The table object is usually

Self reference table , Use one or more of these columns as another column in the same table （ Or a subset of these columns ） The foreign key constraint of .

Hierarchical query operates on several rows , A node within a logical structure in which one or more column values correspond to parent-child relationships . If

The father has many children , There is brotherhood among children of the same parents . for example , These relationships may reflect an organization's

Reporting structures among employees and managers at the Department and management levels .

The syntax supported by this clause is for SQL Of ANSI/ISO Standard extension .

START WITH Clause

Optional START WITH Clause specifies the condition . Rows that meet this condition become enabled in the hierarchical query CONNECT BY Clause

The root of the recursive operation of .

usage

START WITH Clause assignment CONNECT BY Clause for the first iteration of its recursive activity . If you save

A little START WITH Clause , be CONNECT BY Clause takes each row as the root of the hierarchy for the initial set of intermediate results

The reason is .

CONNECT BY Clause

CONNECT BY Clause specifies conditions for executing recursive operations in hierarchical queries .

CONNECT BY Clause is right SQL Of ANSI/ISO Standard extension .

usage

If you include START WITH Clause , The search criteria it specifies should be used to generate the first intermediate result set for the hierarchical query .

This is by those who are in FROM The table specified in Clause satisfies START WITH If the condition is true, the row constitutes .

If omitted START WITH Clause , There is no available START WITH Condition as filter , And the first intermediate result

Set is FROM Clause specifies the complete set of rows in the table .

By applying the CONNECT BY search criteria ,CONNECT BY Clause to generate a continuous intermediate result set , Until the iteration generates

When the result set is empty, the recursive process ends .