GBase 8s V8. 8 SQL Guide: Tutorial - 6.2.2 (1)

6.2.2 External connection
This chapter shows how to SELECT Statement to create and use external connections . Create connection discussion internal connection . Although connected
Then treat two or more connected tables equally , But the external connection is different from the table with two or more connections . The external connection makes it
One of the tables becomes the control table （ Also known as external tables ）, Control other dependent tables （ Also known as internal tables ）.
In an inner connection or a simple connection , The result contains only the combination of rows that meet the connection conditions . Discard rows that do not meet the connection conditions .
In the external connection , The result includes the rows that meet the connection conditions and the rows in the control table （ If no matching... Is found in the dependent table
OK, then these lines will be abandoned ） The combination of . There are no matching rows in the dependent table. The rows of the control table are included in the columns selected from the dependent table
NULL value .
External joins allow you to apply join filters to internal tables before applying join conditions .
Earlier versions of the database server only supported for external connections ANSI-SQL Standard grammatical GBase 8s Expand .
This syntax is still supported . However ,ANSI-SQL The standard syntax is more flexible in creating queries . It is recommended to use
ANSI-SQL Standard syntax to create new queries . No matter what form of syntax you use , It must be used for a single query
All external connections in the block .
Before relying on external connections , Determine whether one or more internal connections are operational . When there is no need for supplementary letters from other forms
Rest time , Internal connections can usually be used .
Limit ： Cannot combine... In the same query block GBase 8s and ANSI Outer join grammar .
Information about the syntax of outer join , see also 《GBase 8s SQL guide ： grammar 》.
External connection grammar GBase 8s Expand  
External connection grammar GBase 8s The extension is used at the beginning of the external connection OUTER keyword . When using GBase 
8s Grammatical time , Must be in WHERE Clause contains connection conditions . Will be GBase 8s When the syntax is used for external connections , Count
The database server supports the following three basic types of external connections ：
• External connection of two tables
• An external connection that simply connects to the third table
• Outer join two tables with the third table
The external connection must have Projection Clause 、FROM Clause and WHERE Clause . Connection condition WHERE Son
Expression in sentence . To convert a simple connection to an external connection , stay FROM Clause before the name of the dependent table
Key word OUTER . As shown later in this section , You can include... Multiple times in the query OUTER keyword .
There is no external connection syntax GBase 8s Extension is equivalent to ANSI Right connection .