A number of different evaluation operators are supported.
Table 1. . Groupings - used with the $group task
| Operator |
Description |
| $avg |
Returns the average value in the specified field.
The $avg operator applies only to numeric data
types. |
| $count |
Returns the number of selected documents. |
| $max |
Returns the highest value in the specified field.
The $max operator applies to any data type that
can be used for max (x) scalar function. |
| $min |
Returns the lowest value in the specified field.
The $min operator applies to any data type that
can be used for min(x) scalar function. |
| $sum |
Returns the sum of all the values in the specified
field. The $sum operator applies only to numeric
data types. |
| $first |
Returns the first value in the specified field. |
| $last |
Returns the last value in the specified field. |
Table 2. . Arithmetic - applies
to numeric data types
| Operator |
Description |
| $add |
Takes an array of numbers and returns the sum
of the values. |
| $divide |
Takes an array of two numbers and returns the
value of the first number that is divided by the second number. |
| $inc |
Takes a number and increases its value by the
specified number. |
| $mod |
Takes a list of two numbers and returns the
remainder of the first number that is divided by the second number. |
| $multiply |
Takes a list of numbers and returns the value
of multiplying the numbers. |
| $subtract |
Takes a list of two numbers and returns the
difference of subtracting the first number from the second number. |
Table 3. . String functions - applies to
string data types
| Operator |
Description |
| $concat |
Takes an array of strings and append the strings
together into a concatenated string. |
| $strcasecmp |
Takes two strings and compares them. Returns
an integer that reflects the comparison. If the returned integer is
positive, the value of first string is greater than the value of the
second string. If the returned integer is negative, the value of first
string is less than the value of the second string. A return of 0
means both strings are of equal value. |
| $substr |
Takes a string and two numbers. The first number
specifies the starting position and the second number specifies the
length of the string to return. |
| $toLower |
Takes a string and returns it in all lowercase
letters. |
| $toUpper |
Takes a string and returns it in all uppercase
letters. |
Table 4. . Date and Time - applies to date
and time data types
| Operator |
Description |
| $dayOfMonth |
Takes a date and returns the day of the month
as a number between 1 and 31. |
| $dayOfWeek |
Takes a date and returns the day of the week
as number between 1 and 7, where 1 represents Sunday and 7 represents
Saturday. |
| $dayOfYear |
Takes a date and returns the day of the year
as a number between 1 and 366. |
| $year |
Takes a date and returns the full value for
the year. |
| $month |
Takes a date and returns the month as a number
between 1 and 12. |
| $week |
Takes a date and returns the week as a number
between 0 and 53. Week 1 is the first week that includes a Thursday. |
| $hour |
Takes a date and returns the hour as a number
0 and 23. |
| $minute |
Takes a date and returns the minute as a number
between 0 and 59. |
| $second |
Takes a date and returns the second as a number
between 0 and 60. The value of 60 occurs only for leap seconds. |
| $millisecond |
Takes a date and returns the millisecond as
a number between 0 and 999. |
Table 5. . Conditional expressions
| Operator |
Description |
| $ifNull |
Takes two expressions and returns the first
expression if it is a non-null value. Otherwise, it returns the second
expression’s value. |