ORACLE table connection

ORACLE table connection details in ORACLE, table connection methods mainly include: internal connection, external connection, self connection: internal connection: This is the most common connection Query

SELECT * from a inner join B ON. ID = B. IDSELECT * from a join B ON. ID = B. IDSELECT * from a, B WHERE. ID = B. ID

 

The query results of the preceding statements are the same, and they are used for inner join. External Connection:

Left join, right join, FULL JOIN

 

During external join queries, you need to understand that the base table and the matching table are all the tables shown in records, and the matching table is the table that displays records with matching conditions, if the record row does not match, the corresponding field is displayed as NULL. Therefore, the number of records displayed is equal to the number of records in the base table.

SELECT * from a left join B on a. ID = B. ID

Table A is A base table, so all records are used. Then, let's look at table B. If records that cannot match Table A through the ID field, NULL is displayed.

SELECT * from a, B where a. ID = B. ID (+)

The effect is the same as that of other links to push the self-join: as the name suggests, the table and its own connection Query