Operators
=========

AQL supports a number of operators that can be used in expressions.  There are
comparison, logical, arithmetic, and the ternary operator.

#### Comparison operators

Comparison (or relational) operators compare two operands. They can be used with
any input data types, and will return a boolean result value.

The following comparison operators are supported:

- *==* equality
- *!=* inequality
- *<*  less than 
- *<=* less or equal
- *>*  greater than
- *>=* greater or equal
- *IN* test if a value is contained in an array
- *NOT IN* test if a value is not contained in an array
- *LIKE* tests if a string value matches a pattern
- *=~* tests if a string value matches a regular expression
- *!~* tests if a string value does not match a regular expression

Each of the comparison operators returns a boolean value if the comparison can
be evaluated and returns *true* if the comparison evaluates to true, and *false*
otherwise. 

The comparison operators accept any data types for the first and second operands. 
However, *IN* and *NOT IN* will only return a meaningful result if their right-hand 
operand is a string, and *LIKE* will only execute if both operands are string values.
The comparison operators will not perform any implicit type casts if the compared 
operands have different or non-sensible types.

Some examples for comparison operations in AQL:

```
0 == null                 // false
1 > 0                     // true
true != null              // true
45 <= "yikes!"            // true
65 != "65"                // true
65 == 65                  // true
1.23 > 1.32               // false
1.5 IN [ 2, 3, 1.5 ]      // true
"foo" IN null             // false
42 NOT IN [ 17, 40, 50 ]  // true
"abc" == "abc"            // true
"abc" == "ABC"            // false
"foo" LIKE "f%"           // true
"foo" =~ "^f[o].$"        // true
"foo" !~ "[a-z]+bar$"     // true
```

The *LIKE* operator checks whether its left operand matches the pattern specified
in its right operand. The pattern can consist of regular characters and wildcards.
The supported wildcards are *_* to match a single arbitrary character, and *%* to 
match any number of arbitrary characters. Literal *%* and *_* need to be escaped
with a backslash. Backslashes need to be escaped themselves, which effectively
means that two reverse solidus characters need to preceed a literal percent sign
or underscore. In arangosh, additional escaping is required, making it four
backslashes in total preceeding the to-be-escaped character.

```
"abc" LIKE "a%"              // true
"abc" LIKE "_bc"             // true
"a_b_foo" LIKE "a\\_b\\_foo" // true
```

The pattern matching performed by the *LIKE* operator is case-sensitive.

The regular expression operators *=~* and *!~* expect their left-hand operands to
be strings, and their right-hand operands to be strings containing valid regular
expressions as specified in the documentation for the AQL function
[REGEX_TEST()](Functions/String.md#regextest).

#### Array comparison operators

The comparison operators also exist as *array variant*. In the array
variant, the operator is prefixed with one of the keywords *ALL*, *ANY*
or *NONE*. Using one of these keywords changes the operator behavior to
execute the comparison operation for all, any, or none of its left hand 
argument values. It is therefore expected that the left hand argument
of an array operator is an array.

Examples:

```
[ 1, 2, 3 ] ALL IN [ 2, 3, 4 ]   // false
[ 1, 2, 3 ] ALL IN [ 1, 2, 3 ]   // true
[ 1, 2, 3 ] NONE IN [ 3 ]        // false
[ 1, 2, 3 ] NONE IN [ 23, 42 ]   // true
[ 1, 2, 3 ] ANY IN [ 4, 5, 6 ]   // false
[ 1, 2, 3 ] ANY IN [ 1, 42 ]     // true
[ 1, 2, 3 ] ANY == 2             // true
[ 1, 2, 3 ] ANY == 4             // false
[ 1, 2, 3 ] ANY > 0              // true
[ 1, 2, 3 ] ANY <= 1             // true
[ 1, 2, 3 ] NONE < 99            // false
[ 1, 2, 3 ] NONE > 10            // true
[ 1, 2, 3 ] ALL > 2              // false
[ 1, 2, 3 ] ALL > 0              // true
[ 1, 2, 3 ] ALL >= 3             // false
["foo", "bar"] ALL != "moo"      // true
["foo", "bar"] NONE == "bar"     // false
["foo", "bar"] ANY == "foo"      // true
```

Note that these operators are not optimized yet. Indexes will not be utilized.

#### Logical operators

The following logical operators are supported in AQL:

- *&&* logical and operator
- *||* logical or operator
- *!* logical not/negation operator

AQL also supports the following alternative forms for the logical operators:

- *AND* logical and operator
- *OR* logical or operator
- *NOT* logical not/negation operator

The alternative forms are aliases and functionally equivalent to the regular 
operators.

The two-operand logical operators in AQL will be executed with short-circuit 
evaluation (except if one of the operands is or includes a subquery. In this
case the subquery will be pulled out an evaluated before the logical operator).

The result of the logical operators in AQL is defined as follows:

- `lhs && rhs` will return `lhs` if it is `false` or would be `false` when converted
  into a boolean. If `lhs` is `true` or would be `true` when converted to a boolean,
  `rhs` will be returned.
- `lhs || rhs` will return `lhs` if it is `true` or would be `true` when converted
  into a boolean. If `lhs` is `false` or would be `false` when converted to a boolean,
  `rhs` will be returned.
- `! value` will return the negated value of `value` converted into a boolean

Some examples for logical operations in AQL:

```js
u.age > 15 && u.address.city != ""
true || false
NOT u.isInvalid
1 || ! 0
```

Passing non-boolean values to a logical operator is allowed. Any non-boolean operands 
will be casted to boolean implicitly by the operator, without making the query abort.

The *conversion to a boolean value* works as follows:
- `null` will be converted to `false`
- boolean values remain unchanged
- all numbers unequal to zero are `true`, zero is `false`
- an empty string is `false`, all other strings are `true`
- arrays (`[ ]`) and objects / documents (`{ }`) are `true`, regardless of their contents

The result of *logical and* and *logical or* operations can now have any data 
type and is not necessarily a boolean value.

For example, the following logical operations will return boolean values:

```js
25 > 1 && 42 != 7                          // true
22 IN [ 23, 42 ] || 23 NOT IN [ 22, 7 ]    // true
25 != 25                                   // false
```

whereas the following logical operations will not return boolean values:

```js
1 || 7                                     // 1
null || "foo"                              // "foo"
null && true                               // null
true && 23                                 // 23
```
   
#### Arithmetic operators

Arithmetic operators perform an arithmetic operation on two numeric
operands. The result of an arithmetic operation is again a numeric value.

AQL supports the following arithmetic operators:

- *+* addition
- *-* subtraction
- <i>\*</i> multiplication
- */* division
- *%* modulus

Unary plus and unary minus are supported as well:

```js
LET x = -5
LET y = 1
RETURN [-x, +y]
// [5, 1]
```

For exponentiation, there is a [numeric function](Functions/Numeric.md#pow) *POW()*.
The syntax `base ** exp` is not supported.

For string concatenation, you must use the [string function](Functions/String.md#concat)
*CONCAT()*. Combining two strings with a plus operator (`"foo" + "bar"`) will not work!
Also see [Common Errors](CommonErrors.md).

Some example arithmetic operations:

```
1 + 1
33 - 99
12.4 * 4.5
13.0 / 0.1
23 % 7
-15
+9.99
```

The arithmetic operators accept operands of any type. Passing non-numeric values to an 
arithmetic operator will cast the operands to numbers using the type casting rules 
applied by the [TO_NUMBER()](Functions/TypeCast.md#tonumber) function:

- `null` will be converted to `0`
- `false` will be converted to `0`, true will be converted to `1`
- a valid numeric value remains unchanged, but NaN and Infinity will be converted to `0`
- string values are converted to a number if they contain a valid string representation
  of a number. Any whitespace at the start or the end of the string is ignored. Strings
  with any other contents are converted to the number `0`
- an empty array is converted to `0`, an array with one member is converted to the numeric
  representation of its sole member. Arrays with more members are converted to the number 
  `0`.
- objects / documents are converted to the number `0`.

An arithmetic operation that produces an invalid value, such as `1 / 0` (division by zero)
will also produce a result value of `null`. The query is not aborted, but you may see a
warning.

Here are a few examples:

```
1 + "a"                 // 1
1 + "99"                // 100
1 + null                // 1
null + 1                // 1
3 + [ ]                 // 3
24 + [ 2 ]              // 26
24 + [ 2, 4 ]           // 0
25 - null               // 25
17 - true               // 16
23 * { }                // 0
5 * [ 7 ]               // 35
24 / "12"               // 2
1 / 0                   // 0
```

#### Ternary operator

AQL also supports a ternary operator that can be used for conditional
evaluation. The ternary operator expects a boolean condition as its first
operand, and it returns the result of the second operand if the condition
evaluates to true, and the third operand otherwise.

*Examples*

```js
u.age > 15 || u.active == true ? u.userId : null
```

There is also a shortcut variant of the ternary operator with just two
operands. This variant can be used when the expression for the boolean
condition and the return value should be the same:

*Examples*

```js
u.value ? : 'value is null, 0 or not present'
```


#### Range operator

AQL supports expressing simple numeric ranges with the *..* operator.
This operator can be used to easily iterate over a sequence of numeric
values.    

The *..* operator will produce an array of values in the defined range, with 
both bounding values included.

*Examples*

```
2010..2013
```

will produce the following result:

```json
[ 2010, 2011, 2012, 2013 ]
```

There is also a [RANGE() function](Functions/Numeric.md#range).

#### Array operators

AQL provides array operators <i>[\*]</i> for
[array variable expansion](Advanced/ArrayOperators.md#array-expansion) and
<i>[\*\*]</i> for [array contraction](Advanced/ArrayOperators.md#array-contraction).

#### Operator precedence

The operator precedence in AQL is similar as in other familiar languages (lowest precedence first):

- *? :* ternary operator
- *||* logical or
- *&&* logical and
- *==*, *!=* equality and inequality
- *IN* in operator
- *<*, *<=*, *>=*, *>* less than, less equal,
  greater equal, greater than
- *+*, *-* addition, subtraction
- <i>\*</i>, */*, *%* multiplication, division, modulus
- *!*, *+*, *-* logical negation, unary plus, unary minus
- <i>[\*]</i> expansion
- *()* function call
- *.* member access
- *[]* indexed value access

The parentheses *(* and *)* can be used to enforce a different operator
evaluation order.