What are the functions and limitations of external connections in Oracle database

Oracle database of Chinese and foreign connections and what are the restrictions, many novices are not very clear about this, in order to help you solve this problem, the following editor will explain in detail for you, people with this need can come to learn, I hope you can gain something.

What are the functions and limitations of external connections in Oracle database

Outer link: even if the column in the connection contains a null value, the outer link returns a row. You can use the outer link operator to perform an outer link in the connection condition; the outer link operator is a plus sign enclosed in parentheses: (+)

It is important to note that although an outer link can be placed on either side of the join operator, it should usually be placed on the opposite side of the column containing null values in the row you want to retrieve.

1. Left outer connection and right outer connection

External links can be divided into two categories: left outer connection and right outer connection. To understand the difference between left outer connection and right outer connection, consider the following syntax:

SELECT.

FROM table1,table2...

Suppose these two tables join table1.column1 and table2.column2: then assume that table1 contains a row with a null value of column1. To perform a left outer join, the WHERE clause is as follows:

WHERE table1.column1=table2.column (+)

In the left outer join, the outer link operator is actually equal to the right side of the operator. In the right outer join, the outer link operator is actually equal to the left side of the operator.

two。 Restrictions on external links

There are some restrictions in the use of external links, such as using the outer join operator only on one end of the outer link, but not on both ends at the same time. If you try to use the outer join operator on both ends of the connection, you will see an error. In addition, you cannot use both an external link condition and an IN operator, nor can you use both an external link condition and another join condition that uses the OR operator.

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