• EntityFunction [x, body] behaves exactly like an EntityProperty and can be used directly in EntityValue .
• In EntityFunction [x,body], x is usually bound to a single Entity at a time. The second argument of AggregatedEntityClass is an exception to this rule, since the formal parameter will bind to each group of entities.
• In the context of the second argument of AggregatedEntityClass for relational-database-backed entities, the following aggregation functions are supported:
• Min minimum of values
  Max maximum of values
  Length number of values
  Total sum of values
  Mean the mean value
  StandardDeviation sample standard deviation
  Variance sample variance
• EntityFunction [{left,right},body] can appear as the condition for CombinedEntityClass . Here, left will bind to each Entity in the first argument of CombinedEntityClass and right to each Entity in the second argument.
• When dealing with entities registered in a relational-database-backed EntityStore , only the following forms can be compiled to SQL:
• a+b additions between numbers, dates and time quantities
  a-b differences between numbers or dates
  a*b multiplications between numbers
  a/b ratios between numbers
  a^b powers between numbers
  Quotient [a,b] integer division
  Mod [a,b] remainder of division between integers
  Sqrt [a] square root
  Exp [a] exponential of numbers
  Log ,Log10 ,Log2 logarithms in different bases
  Abs [a] absolute value
  OddQ [a],EvenQ [a] parity checks on integers
  ab,a>b,a≠b,… comparisons between numbers, dates, strings
  a&&b,a||b,!a,… Boolean algebra
  If [a,b,c] if statements
  Which […] which statements
  MissingQ [a] check whether an element is missing
  Length [list]0 check whether a list is empty
  OrderedQ [a,b] check the alphabetical order of strings
  MemberQ [list,a] check whether an element is in a list or an entity in a class
  StringJoin [a,b,…] string concatenation
  StringMatchQ […],StringStartsQ […],… basic string matching operations
  Now ,Today getting the current date
  AllTrue ,AnyTrue Boolean operations on lists
  SelectFirst [list,Not@*MissingQ ] select the first non-missing value in a list
  BitNot ,BitAnd ,BitOr ,BitShiftLeft ,… bitwise operations
  Sin ,Cos ,Tan ,… trigonometric functions
  ArcSin ,ArcCos ,ArcTan ,… inverse trigonometric functions
  Sinh ,Cosh ,Tanh ,… hyperbolic functions
  ArcSinh ,ArcCosh ,ArcTanh inverse hyperbolic functions
  Round ,Floor ,Ceiling rounding functions
  Min ,Max minimum and maximum (outside aggregations)
  StringTake ,StringDrop extract a subpart of a string (including UpTo support)
  ToString [a] convert a number or date to string
  N [a] convert an integer to real
  IntegerPart [a] extract the integer part of a real number
  FromDigits [a] convert a string to integer
  Boole [a] convert a Boolean to 0 or 1
  Interpreter ["StructuredDate" ][a],Interpreter ["StructuredTime" ][a],Interpreter ["StructuredDateTime" ][a] convert a string to date or time object
  UnixTime [a] convert a date object with time to a number
  FromUnixTime [a] convert a number to a date object with time
  JulianDate [a] convert a date object to a number
  FromJulianDate [a] convert a number to a date object
  DateValue [a,"SecondsFromMidnight"] convert a time object to a number
  DateValue [UnixTime [a],"TimeObject"] convert a number to a time object
• In the context of relational-database-backed entities, the functions above might have limited functionality with respect to their Wolfram Language counterpart. For example, SQL lacks support for complex numbers; because of this, the domains of numerical functions are limited to regions where they are real valued. Symbolic evaluation is generally not available. Precision-tracking semantics may also vary.

Basic Examples  (3)

Properties & Relations  (6)

Possible Issues  (1)