Oracle: What Does '(+)' Do in a Where Clause

Oracle: What does `(+)` do in a WHERE clause?

Depending on which side of the "=" the "(+) is on, it denotes a LEFT OUTER or a RIGHT OUTER join (in this case, it's a left outer join). It's old Oracle syntax that is sometimes preferred by people who learned it first, since they like that it makes their code shorter.

Best not to use it though, for readability's sake.

What does a (+) sign mean in an Oracle SQL WHERE clause?

This is an Oracle-specific notation for an outer join. It means that it will include all rows from t1, and use NULLS in the t0 columns if there is no corresponding row in t0.

In standard SQL one would write:

SELECT t0.foo, t1.bar
  FROM FIRST_TABLE t0
 RIGHT OUTER JOIN SECOND_TABLE t1;

Oracle recommends not to use those joins anymore if your version supports ANSI joins (LEFT/RIGHT JOIN) :

Oracle recommends that you use the FROM clause OUTER JOIN syntax rather than the Oracle join operator. Outer join queries that use the Oracle join operator (+) are subject to the following rules and restrictions […]

Function in Oracle SQL where clause

A function returns a value.

In the where clause you need to compare that return value with something, e.g.

SELECT * 
FROM employees emp 
WHERE emp.loc = 'US' 
AND usr.fnctn('emp_sal', :sal1, :sal2) = 42;

Why are aggregate functions not allowed in where clause

The reason you can't use SUM() in the WHERE clause is the order of evaluation of clauses.

FROM tells you where to read rows from. Right as rows are read from disk to memory, they are checked for the WHERE conditions. (Actually in many cases rows that fail the WHERE clause will not even be read from disk. "Conditions" are formally known as predicates and some predicates are used - by the query execution engine - to decide which rows are read from the base tables. These are called access predicates.) As you can see, the WHERE clause is applied to each row as it is presented to the engine.

On the other hand, aggregation is done only after all rows (that verify all the predicates) have been read.

Think about this: SUM() applies ONLY to the rows that satisfy the WHERE conditions. If you put SUM() in the WHERE clause, you are asking for circular logic. Does a new row pass the WHERE clause? How would I know? If it will pass, then I must include it in the SUM, but if not, it should not be included in the SUM. So how do I even evaluate the SUM condition?

Oracle SQL: Meaning of (+)= in WHERE clause

The (+) identifies the table that is being outer joined to. The way I was taught, the (+) indicated the table that would have missing rows for which new NULL rows had to be added.

If you look at the alternate left outer join syntaxes that various databases supported before LEFT OUTER JOIN became part of the ANSI standard, the proprietary operator was generally applied to the table that was "missing" rows. DB2 also supports the (+) operator for outer joins in the same way that Oracle does.

Answer: Old Style Oracle Outer Join Syntax - Why locate the (+) on the right side of the equals sign in a Left Outer join?

Not equal != operator on NULL

<> is Standard SQL-92; != is its equivalent. Both evaluate for values, which NULL is not -- NULL is a placeholder to say there is the absence of a value.

Which is why you can only use IS NULL/IS NOT NULL as predicates for such situations.

This behavior is not specific to SQL Server. All standards-compliant SQL dialects work the same way.

Note: To compare if your value is not null, you use IS NOT NULL, while to compare with not null value, you use <> 'YOUR_VALUE'. I can't say if my value equals or not equals to NULL, but I can say if my value is NULL or NOT NULL. I can compare if my value is something other than NULL.

FUNCTION in WHERE clause of SQL query

I wanted to make private function returning previous month. Why I cannot use this return value from GET_PREV_MONTH() function in WHERE clause of my other SQL queries

It is a private function - you cannot reference it outside the package.

If you want to use it in SQL then declare it in the package specification so that it is public.

This is private function of this package! This function is used in procedures from the same package. It is not referenced outside this package.

It CANNOT be used in the SQL context if it is not a public function; regardless of whether the SQL is being called from inside or outside the same package.

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