SQL 查询中 (+) 的含义
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Meaning of (+) in SQL queries
提问by Zabbala
I've come across some SQL queries in Oracle that contain '(+)' and I have no idea what that means. Can someone explain its purpose or provide some examples of its use? Thanks
我在 Oracle 中遇到过一些包含“(+)”的 SQL 查询，但我不知道这意味着什么。有人可以解释其目的或提供一些使用示例吗？谢谢
回答by Quassnoi
It's Oracle's synonym for OUTER JOIN.
它是 Oracle 的同义词OUTER JOIN。
SELECT *
FROM a, b
WHERE b.id(+) = a.id
gives same result as
给出相同的结果
SELECT *
FROM a
LEFT OUTER JOIN b
ON b.id = a.id
回答by Geoff
The + is a short cut for OUTER JOIN, depending on which side you put it on, it indicates a LEFT or RIGHT OUTER JOIN
+ 是 OUTER JOIN 的快捷方式，取决于你把它放在哪一边，它表示 LEFT 或 RIGHT OUTER JOIN
Check the second entry in this forum postfor some examples
检查此论坛帖子中的第二个条目以获取一些示例
回答by Harper Shelby
IIRC, the + is used in older versions of Oracle to indicate an outer join in the pre-ANSI SQL join syntax. In other words:
IIRC，在旧版本的 Oracle 中使用 + 来指示预 ANSI SQL 连接语法中的外部连接。换句话说：
select foo,bar
from a, b
where a.id = b.id+
is the equivalent of
相当于
select foo,bar
from a left outer join b
on a.id = b.id
NOTE: this may be backwards/slightly incorrect, as I've never used the pre-ANSI SQL syntax.
注意：这可能是倒退/稍微不正确，因为我从未使用过 ANSI 之前的 SQL 语法。
回答by Cory House
You use this to assure that the table you're joining doesn't reduce the amount of records returned. So it's handy when you're joining to a table that may not have a record for every key you're joining on.
您使用它来确保您加入的表不会减少返回的记录数量。因此，当您加入一个可能没有您加入的每个键的记录的表时，这很方便。
For example, if you were joining a Customer and Purchase table:
例如，如果您要加入 Customer 和 Purchase 表：
To list of allcustomers and all their purchases, you want to do an outer join (+) on the Purchase table so customers that haven't purchased anything still show up in your report.
要列出所有客户及其所有购买，您需要在购买表上执行外连接 (+)，以便尚未购买任何产品的客户仍显示在您的报告中。
