为什么在“WHERE”子句中使用“= +”和“= *”？

Question

I have issue related database connection in java class,i don't know why use this "=+" and "=*" in "WHERE " clause.

Here's an example:

String where = null;
if (isOracleConnectionCache()) {
    where = "ValidInfo.InfoCode = FolderInfo.InfoCode AND ValidInfoGroup.InfoGroup =+ ValidInfo.InfoGroup AND FolderInfo.FolderRSN = ?";
} else {
    where = "ValidInfo.InfoCode = FolderInfo.InfoCode AND ValidInfoGroup.InfoGroup =* ValidInfo.InfoGroup AND FolderInfo.FolderRSN = ?";
}

Can anyone tell me ?

I have three question :

(1) What do "*" and "+" signs denote ?

(2) how do these =+ and =* work in the WHERE clause ?

(3)how it is compare with two table ?

Answer 1

So, as already explained by others, the =* operator in SQL Server indicates an outer join.

However, in Oracle =+ is not an operator at all. It appears that ValidInfoGroup.InfoGroup =+ ValidInfo.InfoGroup is actually parsed as ValidInfoGroup.InfoGroup = (+ ValidInfo.InfoGroup) , where + is the unary identity operator.

Since this code appears to be an attempt to write an outer join depending on which database is being used, it is incorrect when used in Oracle - it is actually doing a normal join. The proper way to write this condition in Oracle's custom syntax would be ValidInfoGroup.InfoGroup = ValidInfo.InfoGroup (+) .

It would perhaps be better to use ANSI SQL join syntax to indicate the outer join, which I believe would remove the need to test which database is being used.

Thanks to Martin and Nicholas for helpful comments on the other answers.

Answer 2

=* does OUTER JOIN in SQL Server

UPDATE

(+) = does OUTER JOIN in Oracle

I dont know what =+ does sorry for the confusion

Answer 3

这只不过是Oracle和MS-SQL中的外部连接让我们假设我删除了一个表记录并且相应的值为NULL所以当你从我的理解中比较==时会产生问题所以我们可以像你提到的那样使用外连接在你的问题。