Translate to NULLIF

EFCore.slnx.DotSettings

@@ -357,6 +357,8 @@ The .NET Foundation licenses this file to you under the MIT license.
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=subquery/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=subquery_0027s/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=transactionality/@EntryIndexedValue">True</s:Boolean>
+	<s:Boolean x:Key="/Default/UserDictionary/Words/=uncoalesce/@EntryIndexedValue">True</s:Boolean>
+	<s:Boolean x:Key="/Default/UserDictionary/Words/=uncoalescing/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=unconfigured/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=unignore/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=fixup/@EntryIndexedValue">True</s:Boolean>

src/EFCore.Cosmos/Query/Internal/SqlExpressionFactory.cs

@@ -707,10 +707,37 @@ public virtual SqlExpression Condition(SqlExpression test, SqlExpression ifTrue,
     {
         var typeMapping = ExpressionExtensions.InferTypeMapping(ifTrue, ifFalse);
 
-        return new SqlConditionalExpression(
-            ApplyTypeMapping(test, _boolTypeMapping),
-            ApplyTypeMapping(ifTrue, typeMapping),
-            ApplyTypeMapping(ifFalse, typeMapping));
+        test = ApplyTypeMapping(test, _boolTypeMapping);
+        ifTrue = ApplyTypeMapping(ifTrue, typeMapping);
+        ifFalse = ApplyTypeMapping(ifFalse, typeMapping);
+
+        // Simplify:
+        //   a == b ? b : a -> a
+        //   a != b ? a : b -> a
+        if (test is SqlBinaryExpression
+            {
+                OperatorType: ExpressionType.Equal or ExpressionType.NotEqual,
+                Left: SqlExpression left,
+                Right: SqlExpression right
+            } binary)
+        {
+            // Swap ifTrue/ifFalse for NotEqual so we can reason uniformly about ifEqual/ifNotEqual.
+            var (ifEqual, ifNotEqual) = binary.OperatorType is ExpressionType.Equal ? (ifTrue, ifFalse) : (ifFalse, ifTrue);
+
+            // 'left' survives: a == b ? b : a -> a
+            if (left.Equals(ifNotEqual) && right.Equals(ifEqual))
+            {
+                return left;
+            }
+
+            // 'right' survives: a == b ? a : b -> b
+            if (right.Equals(ifNotEqual) && left.Equals(ifEqual))
+            {
+                return right;
+            }
+        }
+
+        return new SqlConditionalExpression(test, ifTrue, ifFalse);
     }
 
     /// <summary>

src/EFCore.Relational/Query/SqlExpressionFactory.cs

@@ -837,6 +837,61 @@ public virtual SqlExpression Case(
             elseResult = lastCase.ElseResult;
         }
 
+        // Simplify:
+        //   a == b ? b : a -> a
+        //   a != b ? a : b -> a
+        // And lift:
+        //   a == b ? null : a -> NULLIF(a, b)
+        //   a != b ? a : null -> NULLIF(a, b)
+        if (operand is null
+            && typeMappedWhenClauses is
+            [
+                {
+                    Test: SqlBinaryExpression
+                    {
+                        OperatorType: ExpressionType.Equal or ExpressionType.NotEqual,
+                        Left: var left,
+                        Right: var right
+                    } binary,
+                    Result: var result
+                }
+            ])
+        {
+            // Swap result/elseResult for NotEqual so we can reason uniformly about ifEqual/ifNotEqual.
+            // A missing ELSE clause is equivalent to ELSE NULL, so normalize to a null constant for matching.
+            var (ifEqual, ifNotEqual) = binary.OperatorType is ExpressionType.Equal
+                ? (result, elseResult ?? Constant(null, result.Type, result.TypeMapping))
+                : (elseResult ?? Constant(null, result.Type, result.TypeMapping), result);
+
+            // 'left' survives the conditional.
+            if (left.Equals(ifNotEqual))
+            {
+                switch (ifEqual)
+                {
+                    // a == b ? b : a -> a   (also collapses NULLIF(a, NULL) when both are null constants)
+                    case var _ when ifEqual.Equals(right):
+                        return left;
+                    // a == b ? null : a -> NULLIF(a, b)
+                    case SqlConstantExpression { Value: null }:
+                        return Function("NULLIF", [left, right], nullable: true, Statics.TrueFalse, left.Type, left.TypeMapping);
+                }
+            }
+
+            // 'right' survives the conditional (operand-swapped equivalents of the patterns above).
+            if (right.Equals(ifNotEqual))
+            {
+                switch (ifEqual)
+                {
+                    // a == b ? a : b -> b
+                    case var _ when ifEqual.Equals(left):
+                        return right;
+                    // a == b ? null : b -> NULLIF(b, a)
+                    case SqlConstantExpression { Value: null }:
+                        return Function("NULLIF", [right, left], nullable: true, Statics.TrueFalse, right.Type, right.TypeMapping);
+                }
+            }
+        }
+
         return existingExpression is CaseExpression expr
             && operand == expr.Operand
             && typeMappedWhenClauses.SequenceEqual(expr.WhenClauses)

test/EFCore.Cosmos.FunctionalTests/Query/Translations/Operators/MiscellaneousOperatorTranslationsCosmosTest.cs

@@ -24,6 +24,90 @@ FROM root c
 """);
     }
 
+    public override async Task Conditional_simplifiable_equality()
+    {
+        await base.Conditional_simplifiable_equality();
+
+        AssertSql(
+            """
+SELECT VALUE c
+FROM root c
+WHERE (c["Int"] > 1)
+""");
+    }
+
+    public override async Task Conditional_simplifiable_inequality()
+    {
+        await base.Conditional_simplifiable_inequality();
+
+        AssertSql(
+            """
+SELECT VALUE c
+FROM root c
+WHERE (c["Int"] > 1)
+""");
+    }
+
+    public override async Task Conditional_uncoalesce_with_equality_left()
+    {
+        await base.Conditional_uncoalesce_with_equality_left();
+
+        AssertSql(
+            """
+SELECT VALUE c
+FROM root c
+WHERE (((c["Int"] = 9) ? null : c["Int"]) > 1)
+""");
+    }
+
+    public override async Task Conditional_uncoalesce_with_equality_right()
+    {
+        await base.Conditional_uncoalesce_with_equality_right();
+
+        AssertSql(
+            """
+SELECT VALUE c
+FROM root c
+WHERE (((9 = c["Int"]) ? null : c["Int"]) > 1)
+""");
+    }
+
+    public override async Task Conditional_uncoalesce_with_inequality_left()
+    {
+        await base.Conditional_uncoalesce_with_inequality_left();
+
+        AssertSql(
+            """
+SELECT VALUE c
+FROM root c
+WHERE (((c["Int"] != 9) ? c["Int"] : null) > 1)
+""");
+    }
+
+    public override async Task Conditional_uncoalesce_with_inequality_right()
+    {
+        await base.Conditional_uncoalesce_with_inequality_right();
+
+        AssertSql(
+            """
+SELECT VALUE c
+FROM root c
+WHERE (((9 != c["Int"]) ? c["Int"] : null) > 1)
+""");
+    }
+
+    public override async Task Conditional_uncoalesce_with_string()
+    {
+        await base.Conditional_uncoalesce_with_string();
+
+        AssertSql(
+            """
+SELECT VALUE c
+FROM root c
+WHERE (((c["String"] = "Seattle") ? null : c["String"]) = "London")
+""");
+    }
+
     public override async Task Coalesce()
     {
         await base.Coalesce();

test/EFCore.Relational.Specification.Tests/Query/NullSemanticsQueryTestBase.cs

@@ -711,7 +711,7 @@ public virtual Task Where_equal_with_conditional(bool async)
             async,
             ss => ss.Set<NullSemanticsEntity1>().Where(e => (e.NullableStringA == e.NullableStringB
                     ? e.NullableStringA
-                    : e.NullableStringB)
+                    : e.NullableStringC)
                 == e.NullableStringC).Select(e => e.Id));
 
     [Theory, MemberData(nameof(IsAsyncData))]
@@ -721,7 +721,7 @@ public virtual Task Where_not_equal_with_conditional(bool async)
             ss => ss.Set<NullSemanticsEntity1>().Where(e => e.NullableStringC
                 != (e.NullableStringA == e.NullableStringB
                     ? e.NullableStringA
-                    : e.NullableStringB)).Select(e => e.Id));
+                    : e.NullableStringC)).Select(e => e.Id));
 
     [Theory, MemberData(nameof(IsAsyncData))]
     public virtual Task Where_equal_with_conditional_non_nullable(bool async)

test/EFCore.Specification.Tests/Query/Translations/Operators/MiscellaneousOperatorTranslationsTestBase.cs

@@ -12,6 +12,38 @@ public abstract class MiscellaneousOperatorTranslationsTestBase<TFixture>(TFixtu
     public virtual async Task Conditional()
         => await AssertQuery(ss => ss.Set<BasicTypesEntity>().Where(b => (b.Int == 8 ? b.String : "Foo") == "Seattle"));
 
+    [Fact]
+    public virtual async Task Conditional_simplifiable_equality()
+        // ReSharper disable once MergeConditionalExpression
+        => await AssertQuery(ss => ss.Set<NullableBasicTypesEntity>().Where(x => (x.Int == 9 ? 9 : x.Int) > 1));
+
+    [Fact]
+    public virtual async Task Conditional_simplifiable_inequality()
+        // ReSharper disable once MergeConditionalExpression
+        => await AssertQuery(ss => ss.Set<NullableBasicTypesEntity>().Where(x => (x.Int != 8 ? x.Int : 8) > 1));
+
+    // In relational providers, x == a ? null : x ("un-coalescing conditional") is translated to SQL NULLIF
+
+    [Fact]
+    public virtual async Task Conditional_uncoalesce_with_equality_left()
+        => await AssertQuery(ss => ss.Set<BasicTypesEntity>().Where(x => (x.Int == 9 ? null : x.Int) > 1));
+
+    [Fact]
+    public virtual async Task Conditional_uncoalesce_with_equality_right()
+        => await AssertQuery(ss => ss.Set<BasicTypesEntity>().Where(x => (9 == x.Int ? null : x.Int) > 1));
+
+    [Fact]
+    public virtual async Task Conditional_uncoalesce_with_inequality_left()
+        => await AssertQuery(ss => ss.Set<BasicTypesEntity>().Where(x => (x.Int != 9 ? x.Int : null) > 1));
+
+    [Fact]
+    public virtual async Task Conditional_uncoalesce_with_inequality_right()
+        => await AssertQuery(ss => ss.Set<BasicTypesEntity>().Where(x => (9 != x.Int ? x.Int : null) > 1));
+
+    [Fact]
+    public virtual async Task Conditional_uncoalesce_with_string()
+        => await AssertQuery(ss => ss.Set<BasicTypesEntity>().Where(x => (x.String == "Seattle" ? null : x.String) == "London"));
+
     [Fact]
     public virtual async Task Coalesce()
         => await AssertQuery(ss => ss.Set<NullableBasicTypesEntity>().Where(b => (b.String ?? "Unknown") == "Seattle"));

test/EFCore.SqlServer.FunctionalTests/Query/GearsOfWarQuerySqlServerTest.cs

@@ -873,9 +873,7 @@ public override async Task Select_null_propagation_works_for_multiple_navigation
 
         AssertSql(
             """
-SELECT CASE
-    WHEN [c].[Name] IS NOT NULL THEN [c].[Name]
-END
+SELECT [c].[Name]
 FROM [Tags] AS [t]
 LEFT JOIN [Gears] AS [g] ON [t].[GearNickName] = [g].[Nickname] AND [t].[GearSquadId] = [g].[SquadId]
 LEFT JOIN [Tags] AS [t0] ON ([g].[Nickname] = [t0].[GearNickName] OR ([g].[Nickname] IS NULL AND [t0].[GearNickName] IS NULL)) AND ([g].[SquadId] = [t0].[GearSquadId] OR ([g].[SquadId] IS NULL AND [t0].[GearSquadId] IS NULL))
@@ -2048,10 +2046,7 @@ public override async Task Optional_navigation_type_compensation_works_with_pred
 SELECT [t].[Id], [t].[GearNickName], [t].[GearSquadId], [t].[IssueDate], [t].[Note]
 FROM [Tags] AS [t]
 LEFT JOIN [Gears] AS [g] ON [t].[GearNickName] = [g].[Nickname] AND [t].[GearSquadId] = [g].[SquadId]
-WHERE CASE
-    WHEN [g].[HasSoulPatch] = CAST(1 AS bit) THEN CAST(1 AS bit)
-    ELSE [g].[HasSoulPatch]
-END = CAST(0 AS bit)
+WHERE [g].[HasSoulPatch] = CAST(0 AS bit)
 """);
     }
 
@@ -2064,10 +2059,7 @@ public override async Task Optional_navigation_type_compensation_works_with_pred
 SELECT [t].[Id], [t].[GearNickName], [t].[GearSquadId], [t].[IssueDate], [t].[Note]
 FROM [Tags] AS [t]
 LEFT JOIN [Gears] AS [g] ON [t].[GearNickName] = [g].[Nickname] AND [t].[GearSquadId] = [g].[SquadId]
-WHERE CASE
-    WHEN [g].[HasSoulPatch] = CAST(0 AS bit) THEN CAST(0 AS bit)
-    ELSE [g].[HasSoulPatch]
-END = CAST(0 AS bit)
+WHERE [g].[HasSoulPatch] = CAST(0 AS bit)
 """);
     }
 
@@ -3114,9 +3106,7 @@ public override async Task Select_null_conditional_with_inheritance(bool async)
 
         AssertSql(
             """
-SELECT CASE
-    WHEN [f].[CommanderName] IS NOT NULL THEN [f].[CommanderName]
-END
+SELECT [f].[CommanderName]
 FROM [Factions] AS [f]
 """);
     }

test/EFCore.SqlServer.FunctionalTests/Query/Inheritance/TPCGearsOfWarQuerySqlServerTest.cs

@@ -1175,9 +1175,7 @@ public override async Task Select_null_propagation_works_for_multiple_navigation
 
         AssertSql(
             """
-SELECT CASE
-    WHEN [c].[Name] IS NOT NULL THEN [c].[Name]
-END
+SELECT [c].[Name]
 FROM [Tags] AS [t]
 LEFT JOIN (
     SELECT [g].[Nickname], [g].[SquadId]
@@ -2822,10 +2820,7 @@ UNION ALL
     SELECT [o].[Nickname], [o].[SquadId], [o].[HasSoulPatch]
     FROM [Officers] AS [o]
 ) AS [u] ON [t].[GearNickName] = [u].[Nickname] AND [t].[GearSquadId] = [u].[SquadId]
-WHERE CASE
-    WHEN [u].[HasSoulPatch] = CAST(1 AS bit) THEN CAST(1 AS bit)
-    ELSE [u].[HasSoulPatch]
-END = CAST(0 AS bit)
+WHERE [u].[HasSoulPatch] = CAST(0 AS bit)
 """);
     }
 
@@ -2844,10 +2839,7 @@ UNION ALL
     SELECT [o].[Nickname], [o].[SquadId], [o].[HasSoulPatch]
     FROM [Officers] AS [o]
 ) AS [u] ON [t].[GearNickName] = [u].[Nickname] AND [t].[GearSquadId] = [u].[SquadId]
-WHERE CASE
-    WHEN [u].[HasSoulPatch] = CAST(0 AS bit) THEN CAST(0 AS bit)
-    ELSE [u].[HasSoulPatch]
-END = CAST(0 AS bit)
+WHERE [u].[HasSoulPatch] = CAST(0 AS bit)
 """);
     }
 
@@ -4143,9 +4135,7 @@ public override async Task Select_null_conditional_with_inheritance(bool async)
 
         AssertSql(
             """
-SELECT CASE
-    WHEN [l].[CommanderName] IS NOT NULL THEN [l].[CommanderName]
-END
+SELECT [l].[CommanderName]
 FROM [LocustHordes] AS [l]
 """);
     }

test/EFCore.SqlServer.FunctionalTests/Query/Inheritance/TPTGearsOfWarQuerySqlServerTest.cs

@@ -1053,9 +1053,7 @@ public override async Task Select_null_propagation_works_for_multiple_navigation
 
         AssertSql(
             """
-SELECT CASE
-    WHEN [c].[Name] IS NOT NULL THEN [c].[Name]
-END
+SELECT [c].[Name]
 FROM [Tags] AS [t]
 LEFT JOIN (
     SELECT [g].[Nickname], [g].[SquadId]
@@ -2434,10 +2432,7 @@ LEFT JOIN (
     SELECT [g].[Nickname], [g].[SquadId], [g].[HasSoulPatch]
     FROM [Gears] AS [g]
 ) AS [s] ON [t].[GearNickName] = [s].[Nickname] AND [t].[GearSquadId] = [s].[SquadId]
-WHERE CASE
-    WHEN [s].[HasSoulPatch] = CAST(1 AS bit) THEN CAST(1 AS bit)
-    ELSE [s].[HasSoulPatch]
-END = CAST(0 AS bit)
+WHERE [s].[HasSoulPatch] = CAST(0 AS bit)
 """);
     }
 
@@ -2453,10 +2448,7 @@ LEFT JOIN (
     SELECT [g].[Nickname], [g].[SquadId], [g].[HasSoulPatch]
     FROM [Gears] AS [g]
 ) AS [s] ON [t].[GearNickName] = [s].[Nickname] AND [t].[GearSquadId] = [s].[SquadId]
-WHERE CASE
-    WHEN [s].[HasSoulPatch] = CAST(0 AS bit) THEN CAST(0 AS bit)
-    ELSE [s].[HasSoulPatch]
-END = CAST(0 AS bit)
+WHERE [s].[HasSoulPatch] = CAST(0 AS bit)
 """);
     }
 
@@ -3572,9 +3564,7 @@ public override async Task Select_null_conditional_with_inheritance(bool async)
 
         AssertSql(
             """
-SELECT CASE
-    WHEN [l].[CommanderName] IS NOT NULL THEN [l].[CommanderName]
-END
+SELECT [l].[CommanderName]
 FROM [Factions] AS [f]
 LEFT JOIN [LocustHordes] AS [l] ON [f].[Id] = [l].[Id]
 WHERE [l].[Id] IS NOT NULL