Add NULLIF expression function [FEATURE]

resources/function_help/json/nullif

@@ -0,0 +1,11 @@
+{
+  "name": "nullif",
+  "type": "function",
+  "description": "Returns a null value if value1 equals value2; otherwise it returns value1. This can be used to conditionally substitute values with NULL.",
+  "arguments": [ {"arg":"value1", "description": "The value that should either be used or substituted with NULL."},
+                 {"arg":"value2", "description": "The control value that will trigger the NULL substitution."}],
+  "examples": [ { "expression":"nullif('(none)', '(none)')", "returns":"NULL"},
+                { "expression":"nullif('text', '(none)')", "returns":"'text'"},
+                { "expression":"nullif(\"name\", ''), "returns":"NULL, if name is an empty string (or already NULL), the name in any other case."} ]
+}
+

src/core/expression/qgsexpressionfunction.cpp

@@ -947,6 +947,18 @@ static QVariant fcnCoalesce( const QVariantList &values, const QgsExpressionCont
   }
   return QVariant();
 }
+
+static QVariant fcnNullIf( const QVariantList &values, const QgsExpressionContext *, QgsExpression *, const QgsExpressionNodeFunction * )
+{
+  const QVariant val1 = values.at( 0 );
+  const QVariant val2 = values.at( 1 );
+
+  if ( val1 == val2 )
+    return QVariant();
+  else
+    return val1;
+}
+
 static QVariant fcnLower( const QVariantList &values, const QgsExpressionContext *, QgsExpression *parent, const QgsExpressionNodeFunction * )
 {
   QString str = QgsExpressionUtils::getStringValue( values.at( 0 ), parent );
@@ -4575,6 +4587,7 @@ const QList<QgsExpressionFunction *> &QgsExpression::Functions()
         << new QgsStaticExpressionFunction( QStringLiteral( "to_dm" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "value" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "axis" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "precision" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "formatting" ), true ), fcnToDegreeMinute, QStringLiteral( "Conversions" ), QString(), false, QSet<QString>(), false, QStringList() << QStringLiteral( "todm" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "to_dms" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "value" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "axis" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "precision" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "formatting" ), true ), fcnToDegreeMinuteSecond, QStringLiteral( "Conversions" ), QString(), false, QSet<QString>(), false, QStringList() << QStringLiteral( "todms" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "coalesce" ), -1, fcnCoalesce, QStringLiteral( "Conditionals" ), QString(), false, QSet<QString>(), false, QStringList(), true )
+        << new QgsStaticExpressionFunction( QStringLiteral( "nullif" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "value1" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "value2" ) ), fcnNullIf, QStringLiteral( "Conditionals" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "if" ), 3, fcnIf, QStringLiteral( "Conditionals" ), QString(), false, QSet<QString>(), true )
 
         << new QgsStaticExpressionFunction( QStringLiteral( "aggregate" ),

tests/src/core/testqgsexpression.cpp

@@ -1129,6 +1129,11 @@ class TestQgsExpression: public QObject
       QTest::newRow( "coalesce null" ) << "coalesce(NULL)" << false << QVariant();
       QTest::newRow( "coalesce mid-null" ) << "coalesce(1, NULL, 3)" << false << QVariant( 1 );
       QTest::newRow( "coalesce exp" ) << "coalesce(NULL, 1+1)" << false << QVariant( 2 );
+      QTest::newRow( "nullif no substitution" ) << "nullif(3, '(none)')" << false << QVariant( 3 );
+      QTest::newRow( "nullif NULL" ) << "nullif(NULL, '(none)')" << false << QVariant();
+      QTest::newRow( "nullif substitute string" ) << "nullif('(none)', '(none)')" << false << QVariant();
+      QTest::newRow( "nullif substitute double" ) << "nullif(3.3, 3.3)" << false << QVariant();
+      QTest::newRow( "nullif substitute int" ) << "nullif(0, 0)" << false << QVariant();
       QTest::newRow( "regexp match" ) << "regexp_match('abc','.b.')" << false << QVariant( 1 );
       QTest::newRow( "regexp match invalid" ) << "regexp_match('abc DEF','[[[')" << true << QVariant();
       QTest::newRow( "regexp match escaped" ) << "regexp_match('abc DEF','\\\\s[A-Z]+')" << false << QVariant( 4 );
@@ -1552,7 +1557,7 @@ class TestQgsExpression: public QObject
       if ( featureMatched )
       {
         QgsFeature feat = res.value<QgsFeature>();
-        QCOMPARE( feat.id(), ( long long )featureId );
+        QCOMPARE( feat.id(), static_cast<QgsFeatureId>( featureId ) );
       }
     }