diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/decimalFunctions.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/decimalFunctions.scala
index f7df2340edb6c..21f8c812c9ce5 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/decimalFunctions.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/decimalFunctions.scala
@@ -38,11 +38,7 @@ case class UnscaledValue(child: Expression) extends UnaryExpression {
   }
 
   override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = {
-    val eval = child.gen(ctx)
-    eval.code + s"""
-      boolean ${ev.nullTerm} = ${eval.nullTerm};
-      long ${ev.primitiveTerm} = ${ev.nullTerm} ? -1 : ${eval.primitiveTerm}.toUnscaledLong();
-     """
+    defineCodeGen(ctx, ev, c => s"$c.toUnscaledLong()")
   }
 }
 
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/predicates.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/predicates.scala
index 846fc9d90a86b..d69324acf0e5a 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/predicates.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/predicates.scala
@@ -146,9 +146,12 @@ case class And(left: Expression, right: Expression)
       }
     }
   }
+
   override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = {
     val eval1 = left.gen(ctx)
     val eval2 = right.gen(ctx)
+
+    // The result should be `false`, if any of them is `false` whenever the other is null or not.
     s"""
       ${eval1.code}
       boolean ${ev.nullTerm} = false;
@@ -192,20 +195,21 @@ case class Or(left: Expression, right: Expression)
       }
     }
   }
+
   override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = {
     val eval1 = left.gen(ctx)
     val eval2 = right.gen(ctx)
+
+    // The result should be `true`, if any of them is `true` whenever the other is null or not.
     s"""
       ${eval1.code}
       boolean ${ev.nullTerm} = false;
-      boolean ${ev.primitiveTerm} = false;
+      boolean ${ev.primitiveTerm} = true;
 
       if (!${eval1.nullTerm} && ${eval1.primitiveTerm}) {
-        ${ev.primitiveTerm} = true;
       } else {
         ${eval2.code}
         if (!${eval2.nullTerm} && ${eval2.primitiveTerm}) {
-          ${ev.primitiveTerm} = true;
         } else if (!${eval1.nullTerm} && !${eval2.nullTerm}) {
           ${ev.primitiveTerm} = false;
         } else {
@@ -218,19 +222,6 @@ case class Or(left: Expression, right: Expression)
 
 abstract class BinaryComparison extends BinaryExpression with Predicate {
   self: Product =>
-  override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = {
-    left.dataType match {
-      case dt: NumericType if ctx.isNativeType(dt) => defineCodeGen (ctx, ev, {
-        (c1, c3) => s"$c1 $symbol $c3"
-      })
-      case TimestampType =>
-        // java.sql.Timestamp does not have compare()
-        super.genCode(ctx, ev)
-      case other => defineCodeGen (ctx, ev, {
-        (c1, c2) => s"$c1.compare($c2) $symbol 0"
-      })
-    }
-  }
 
   override def checkInputDataTypes(): TypeCheckResult = {
     if (left.dataType != right.dataType) {
@@ -258,6 +249,20 @@ abstract class BinaryComparison extends BinaryExpression with Predicate {
     }
   }
 
+  override def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code = {
+    left.dataType match {
+      case dt: NumericType if ctx.isNativeType(dt) => defineCodeGen (ctx, ev, {
+        (c1, c3) => s"$c1 $symbol $c3"
+      })
+      case TimestampType =>
+        // java.sql.Timestamp does not have compare()
+        super.genCode(ctx, ev)
+      case other => defineCodeGen (ctx, ev, {
+        (c1, c2) => s"$c1.compare($c2) $symbol 0"
+      })
+    }
+  }
+
   protected def evalInternal(evalE1: Any, evalE2: Any): Any =
     sys.error(s"BinaryComparisons must override either eval or evalInternal")
 }
@@ -389,9 +394,9 @@ case class If(predicate: Expression, trueValue: Expression, falseValue: Expressi
     val falseEval = falseValue.gen(ctx)
 
     s"""
+      ${condEval.code}
       boolean ${ev.nullTerm} = false;
       ${ctx.primitiveType(dataType)} ${ev.primitiveTerm} = ${ctx.defaultValue(dataType)};
-      ${condEval.code}
       if (!${condEval.nullTerm} && ${condEval.primitiveTerm}) {
         ${trueEval.code}
         ${ev.nullTerm} = ${trueEval.nullTerm};
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/sets.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/sets.scala
index a0c81473ec050..40107c5985481 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/sets.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/sets.scala
@@ -112,11 +112,11 @@ case class AddItemToSet(item: Expression, set: Expression) extends Expression {
         val htype = ctx.primitiveType(dataType)
 
         ev.nullTerm = "false"
+        ev.primitiveTerm = setEval.primitiveTerm
         itemEval.code + setEval.code +  s"""
           if (!${itemEval.nullTerm} && !${setEval.nullTerm}) {
            (($htype)${setEval.primitiveTerm}).add(${itemEval.primitiveTerm});
           }
-          ${htype} ${ev.primitiveTerm} = ($htype)${setEval.primitiveTerm};
          """
       case _ => super.genCode(ctx, ev)
     }
@@ -147,10 +147,8 @@ case class CombineSets(left: Expression, right: Expression) extends BinaryExpres
           val rightValue = iterator.next()
           leftEval.add(rightValue)
         }
-        leftEval
-      } else {
-        null
       }
+      leftEval
     } else {
       null
     }
@@ -164,10 +162,12 @@ case class CombineSets(left: Expression, right: Expression) extends BinaryExpres
         val rightEval = right.gen(ctx)
         val htype = ctx.primitiveType(dataType)
 
-        ev.nullTerm = "false"
+        ev.nullTerm = leftEval.nullTerm
+        ev.primitiveTerm = leftEval.primitiveTerm
         leftEval.code + rightEval.code + s"""
-          ${htype} ${ev.primitiveTerm} = (${htype})${leftEval.primitiveTerm};
-          ${ev.primitiveTerm}.union((${htype})${rightEval.primitiveTerm});
+          if (!${leftEval.nullTerm} && !${rightEval.nullTerm}) {
+            ${leftEval.primitiveTerm}.union((${htype})${rightEval.primitiveTerm});
+          }
         """
       case _ => super.genCode(ctx, ev)
     }