
Comparison Functions and Operators
==================================

Comparison Operators
--------------------

| Operator | Description                                 |
| :------- | :------------------------------------------ |
| `<`      | Less than                                   |
| `>`      | Greater than                                |
| `<=`     | Less than or equal to                       |
| `>=`     | Greater than or equal to                    |
| `=`      | Equal                                       |
| `<>`     | Not equal                                   |
| `!=`     | Not equal (non-standard but popular syntax) |

Range Operator: BETWEEN
-----------------------

The `BETWEEN` operator tests if a value is within a specified range. It uses the syntax `value BETWEEN min AND max`:

    SELECT 3 BETWEEN 2 AND 6;

The statement shown above is equivalent to the following statement:

    SELECT 3 >= 2 AND 3 <= 6;

To test if a value does not fall within the specified range use `NOT BETWEEN`:

    SELECT 3 NOT BETWEEN 2 AND 6;

The statement shown above is equivalent to the following statement:

    SELECT 3 < 2 OR 3 > 6;

A `NULL` in a `BETWEEN` or `NOT BETWEEN` statement is evaluated using the standard `NULL` evaluation rules applied to the equivalent expression above:

    SELECT NULL BETWEEN 2 AND 4; -- null
    
    SELECT 2 BETWEEN NULL AND 6; -- null
    
    SELECT 2 BETWEEN 1 AND NULL; -- false
    
    SELECT 8 BETWEEN NULL AND 6; -- false

The `BETWEEN` and `NOT BETWEEN` operators can also be used to evaluate any orderable type. For example, a `VARCHAR`:

    SELECT 'Paul' BETWEEN 'John' AND 'Ringo'; -- true

Not that the value, min, and max parameters to `BETWEEN` and `NOT BETWEEN` must be the same type. For example, openLooKeng will produce an error if you ask it if John is between 2.3 and 35.2.

IS NULL and IS NOT NULL
-----------------------

The `IS NULL` and `IS NOT NULL` operators test whether a value is null (undefined). Both operators work for all data types. 

Using `NULL` with `IS NULL` evaluates to true:

    select NULL IS NULL; -- true

But any other constant does not:

    SELECT 3.0 IS NULL; -- false

IS DISTINCT FROM and IS NOT DISTINCT FROM
-----------------------------------------

In SQL a `NULL` value signifies an unknown value, so any comparison involving a `NULL` will produce `NULL`. The `IS DISTINCT FROM` and `IS NOT DISTINCT FROM` operators treat `NULL` as a known value and both
operators guarantee either a true or false outcome even in the presence of `NULL` input:

    SELECT NULL IS DISTINCT FROM NULL; -- false
    
    SELECT NULL IS NOT DISTINCT FROM NULL; -- true

In the example shown above, a `NULL` value is not considered distinct from `NULL`. When you are comparing values which may include `NULL` use these operators to guarantee either a `TRUE` or `FALSE` result.

The following truth table demonstrate the handling of `NULL` in `IS DISTINCT FROM` and `IS NOT DISTINCT FROM`:

| a      | b      | a = b   | a <> b  | a DISTINCT b | a NOT DISTINCT b |
| :----- | :----- | :------ | :------ | :----------- | :--------------- |
| `1`    | `1`    | `TRUE`  | `FALSE` | `FALSE`      | `TRUE`           |
| `1`    | `2`    | `FALSE` | `TRUE`  | `TRUE`       | `FALSE`          |
| `1`    | `NULL` | `NULL`  | `NULL`  | `TRUE`       | `FALSE`          |
| `NULL` | `NULL` | `NULL`  | `NULL`  | `FALSE`      | `TRUE`           |

GREATEST and LEAST
------------------

These functions are not in the SQL standard, but are a common extension. Like most other functions in openLooKeng, they return null if any argument is  null. Note that in some other databases, such as PostgreSQL, they only return null if all arguments are null.

The following types are supported: `DOUBLE`, `BIGINT`, `VARCHAR`, `TIMESTAMP`, `TIMESTAMP WITH TIME ZONE`, `DATE`

**greatest(value1, value2, \..., valueN)** -\> \[same as input\]

Returns the largest of the provided values.

**least(value1, value2, \..., valueN)** -\> \[same as input\]

Returns the smallest of the provided values.


Quantified Comparison Predicates: ALL, ANY and SOME
---------------------------------------------------

The `ALL`, `ANY` and `SOME` quantifiers can be used together with comparison operators in the following way:

``` 
expression operator quantifier ( subquery )
```

For example:

    SELECT 'hello' = ANY (VALUES 'hello', 'world'); -- true
    
    SELECT 21 < ALL (VALUES 19, 20, 21); -- false
    
    SELECT 42 >= SOME (SELECT 41 UNION ALL SELECT 42 UNION ALL SELECT 43); -- true

Here are the meanings of some quantifier and comparison operator combinations:

| Expression       | Meaning                                                      |
| :--------------- | :----------------------------------------------------------- |
| `A = ALL (...)`  | Evaluates to `true` when `A` is equal to all values.         |
| `A <> ALL (...)` | Evaluates to `true` when `A` doesn’t match any value.        |
| `A < ALL (...)`  | Evaluates to `true` when `A` is smaller than the smallest value. |
| `A = ANY (...)`  | Evaluates to `true` when `A` is equal to any of the values. This form is equivalent to `A IN (...)`. |
| `A <> ANY (...)` | Evaluates to `true` when `A` doesn’t match one or more values. |
| `A < ANY (...)`  | Evaluates to `true` when `A` is smaller than the biggest value. |