Refs #28643 -- Reorganized database functions.

Thanks Tim Graham for the review.

django/db/models/functions/comparison.py

@@ -0,0 +1,84 @@
+"""Database functions that do comparisons or type conversions."""
+from django.db.models import Func
+
+
+class Cast(Func):
+    """Coerce an expression to a new field type."""
+    function = 'CAST'
+    template = '%(function)s(%(expressions)s AS %(db_type)s)'
+
+    def __init__(self, expression, output_field):
+        super().__init__(expression, output_field=output_field)
+
+    def as_sql(self, compiler, connection, **extra_context):
+        extra_context['db_type'] = self.output_field.cast_db_type(connection)
+        return super().as_sql(compiler, connection, **extra_context)
+
+    def as_postgresql(self, compiler, connection):
+        # CAST would be valid too, but the :: shortcut syntax is more readable.
+        return self.as_sql(compiler, connection, template='%(expressions)s::%(db_type)s')
+
+
+class Coalesce(Func):
+    """Return, from left to right, the first non-null expression."""
+    function = 'COALESCE'
+
+    def __init__(self, *expressions, **extra):
+        if len(expressions) < 2:
+            raise ValueError('Coalesce must take at least two expressions')
+        super().__init__(*expressions, **extra)
+
+    def as_oracle(self, compiler, connection):
+        # Oracle prohibits mixing TextField (NCLOB) and CharField (NVARCHAR2),
+        # so convert all fields to NCLOB when that type is expected.
+        if self.output_field.get_internal_type() == 'TextField':
+            class ToNCLOB(Func):
+                function = 'TO_NCLOB'
+
+            expressions = [
+                ToNCLOB(expression) for expression in self.get_source_expressions()
+            ]
+            clone = self.copy()
+            clone.set_source_expressions(expressions)
+            return super(Coalesce, clone).as_sql(compiler, connection)
+        return self.as_sql(compiler, connection)
+
+
+class Greatest(Func):
+    """
+    Return the maximum expression.
+
+    If any expression is null the return value is database-specific:
+    On PostgreSQL, the maximum not-null expression is returned.
+    On MySQL, Oracle, and SQLite, if any expression is null, null is returned.
+    """
+    function = 'GREATEST'
+
+    def __init__(self, *expressions, **extra):
+        if len(expressions) < 2:
+            raise ValueError('Greatest must take at least two expressions')
+        super().__init__(*expressions, **extra)
+
+    def as_sqlite(self, compiler, connection):
+        """Use the MAX function on SQLite."""
+        return super().as_sqlite(compiler, connection, function='MAX')
+
+
+class Least(Func):
+    """
+    Return the minimum expression.
+
+    If any expression is null the return value is database-specific:
+    On PostgreSQL, return the minimum not-null expression.
+    On MySQL, Oracle, and SQLite, if any expression is null, return null.
+    """
+    function = 'LEAST'
+
+    def __init__(self, *expressions, **extra):
+        if len(expressions) < 2:
+            raise ValueError('Least must take at least two expressions')
+        super().__init__(*expressions, **extra)
+
+    def as_sqlite(self, compiler, connection):
+        """Use the MIN function on SQLite."""
+        return super().as_sqlite(compiler, connection, function='MIN')