diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/mathExpressions.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/mathExpressions.scala
index 615256243ae2..2822e25021f5 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/mathExpressions.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/mathExpressions.scala
@@ -227,7 +227,8 @@ case class Ceil(child: Expression) extends UnaryMathExpression(math.ceil, "CEIL"
   override def dataType: DataType = child.dataType match {
     case dt @ DecimalType.Fixed(_, 0) => dt
     case DecimalType.Fixed(precision, scale) =>
-      DecimalType.bounded(precision - scale + 1, 0)
+      val boundedPrecision = if (precision < scale) 1 else precision - scale + 1
+      DecimalType.bounded(boundedPrecision, 0)
     case _ => LongType
   }
 
@@ -344,7 +345,8 @@ case class Floor(child: Expression) extends UnaryMathExpression(math.floor, "FLO
   override def dataType: DataType = child.dataType match {
     case dt @ DecimalType.Fixed(_, 0) => dt
     case DecimalType.Fixed(precision, scale) =>
-      DecimalType.bounded(precision - scale + 1, 0)
+      val boundedPrecision = if (precision < scale) 1 else precision - scale + 1
+      DecimalType.bounded(boundedPrecision, 0)
     case _ => LongType
   }
 
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/types/Decimal.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/types/Decimal.scala
index 80916ee9c537..9c8ed6b4e02e 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/types/Decimal.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/types/Decimal.scala
@@ -384,13 +384,15 @@ final class Decimal extends Ordered[Decimal] with Serializable {
   def abs: Decimal = if (this.compare(Decimal.ZERO) < 0) this.unary_- else this
 
   def floor: Decimal = if (scale == 0) this else {
-    val newPrecision = DecimalType.bounded(precision - scale + 1, 0).precision
+    val boundedPrecision = if (precision < scale) 1 else precision - scale + 1
+    val newPrecision = DecimalType.bounded(boundedPrecision, 0).precision
     toPrecision(newPrecision, 0, ROUND_FLOOR).getOrElse(
       throw new AnalysisException(s"Overflow when setting precision to $newPrecision"))
   }
 
   def ceil: Decimal = if (scale == 0) this else {
-    val newPrecision = DecimalType.bounded(precision - scale + 1, 0).precision
+    val boundedPrecision = if (precision < scale) 1 else precision - scale + 1
+    val newPrecision = DecimalType.bounded(boundedPrecision, 0).precision
     toPrecision(newPrecision, 0, ROUND_CEILING).getOrElse(
       throw new AnalysisException(s"Overflow when setting precision to $newPrecision"))
   }
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/MathExpressionsSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/MathExpressionsSuite.scala
index 6af0cde73538..414f789f2efb 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/MathExpressionsSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/MathExpressionsSuite.scala
@@ -254,7 +254,7 @@ class MathExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
     testUnary(Ceil, (d: Double) => math.ceil(d).toLong)
     checkConsistencyBetweenInterpretedAndCodegen(Ceil, DoubleType)
 
-    testUnary(Ceil, (d: Decimal) => d.ceil, (-20 to 20).map(x => Decimal(x * 0.1)))
+    testUnary(Ceil, (d: Decimal) => d.ceil, (-20 to 20).map(x => Decimal(x * 0.0001)))
     checkConsistencyBetweenInterpretedAndCodegen(Ceil, DecimalType(25, 3))
     checkConsistencyBetweenInterpretedAndCodegen(Ceil, DecimalType(25, 0))
     checkConsistencyBetweenInterpretedAndCodegen(Ceil, DecimalType(5, 0))
@@ -274,7 +274,7 @@ class MathExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
     testUnary(Floor, (d: Double) => math.floor(d).toLong)
     checkConsistencyBetweenInterpretedAndCodegen(Floor, DoubleType)
 
-    testUnary(Floor, (d: Decimal) => d.floor, (-20 to 20).map(x => Decimal(x * 0.1)))
+    testUnary(Floor, (d: Decimal) => d.floor, (-20 to 20).map(x => Decimal(x * 0.0001)))
     checkConsistencyBetweenInterpretedAndCodegen(Floor, DecimalType(25, 3))
     checkConsistencyBetweenInterpretedAndCodegen(Floor, DecimalType(25, 0))
     checkConsistencyBetweenInterpretedAndCodegen(Floor, DecimalType(5, 0))
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala
index 41e9e2c92ca8..c6c655251ecc 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala
@@ -2635,6 +2635,18 @@ class SQLQuerySuite extends QueryTest with SharedSQLContext {
     }
   }
 
+  test("SPARK-20211: should be able to floor or ceil with a decimal when its precision < scale") {
+    val df = Seq(0).toDF("a")
+    withTempView("tb") {
+      df.createOrReplaceTempView("tb")
+      checkAnswer(sql("SELECT 1 > 0.00001 FROM tb"), Row(true))
+      checkAnswer(sql("SELECT floor(0.0001) FROM tb"), Row(0))
+      checkAnswer(sql("SELECT ceil(0.0001) FROM tb"), Row(1))
+      checkAnswer(sql("SELECT floor(0.00123) FROM tb"), Row(0))
+      checkAnswer(sql("SELECT floor(0.00010) FROM tb"), Row(0))
+    }
+  }
+
   test("SPARK-12868: Allow adding jars from hdfs ") {
     val jarFromHdfs = "hdfs://doesnotmatter/test.jar"
     val jarFromInvalidFs = "fffs://doesnotmatter/test.jar"