[SPARK-21007][SQL] Add SQL function - RIGHT && LEFT

## What changes were proposed in this pull request?
 Add  SQL function - RIGHT && LEFT, same as MySQL:
https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_left
https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_right

## How was this patch tested?
unit test

Author: liuxian <liu.xian3@zte.com.cn>

Closes #18228 from 10110346/lx-wip-0607.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala

@@ -346,6 +346,8 @@ object FunctionRegistry {
     expression[StringSplit]("split"),
     expression[Substring]("substr"),
     expression[Substring]("substring"),
+    expression[Left]("left"),
+    expression[Right]("right"),
     expression[SubstringIndex]("substring_index"),
     expression[StringTranslate]("translate"),
     expression[StringTrim]("trim"),

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringExpressions.scala

@@ -1198,6 +1198,49 @@ case class Substring(str: Expression, pos: Expression, len: Expression)
   }
 }
 
+/**
+ * Returns the rightmost n characters from the string.
+ */
+// scalastyle:off line.size.limit
+@ExpressionDescription(
+  usage = "_FUNC_(str, len) - Returns the rightmost `len`(`len` can be string type) characters from the string `str`,if `len` is less or equal than 0 the result is an empty string.",
+  extended = """
+    Examples:
+      > SELECT _FUNC_('Spark SQL', 3);
+       SQL
+  """)
+// scalastyle:on line.size.limit
+case class Right(str: Expression, len: Expression, child: Expression) extends RuntimeReplaceable {
+  def this(str: Expression, len: Expression) = {
+    this(str, len, If(IsNull(str), Literal(null, StringType), If(LessThanOrEqual(len, Literal(0)),
+      Literal(UTF8String.EMPTY_UTF8, StringType), new Substring(str, UnaryMinus(len)))))
+  }
+
+  override def flatArguments: Iterator[Any] = Iterator(str, len)
+  override def sql: String = s"$prettyName(${str.sql}, ${len.sql})"
+}
+
+/**
+ * Returns the leftmost n characters from the string.
+ */
+// scalastyle:off line.size.limit
+@ExpressionDescription(
+  usage = "_FUNC_(str, len) - Returns the leftmost `len`(`len` can be string type) characters from the string `str`,if `len` is less or equal than 0 the result is an empty string.",
+  extended = """
+    Examples:
+      > SELECT _FUNC_('Spark SQL', 3);
+       Spa
+  """)
+// scalastyle:on line.size.limit
+case class Left(str: Expression, len: Expression, child: Expression) extends RuntimeReplaceable {
+  def this(str: Expression, len: Expression) = {
+    this(str, len, Substring(str, Literal(1), len))
+  }
+
+  override def flatArguments: Iterator[Any] = Iterator(str, len)
+  override def sql: String = s"$prettyName(${str.sql}, ${len.sql})"
+}
+
 /**
  * A function that returns the char length of the given string expression or
  * number of bytes of the given binary expression.

sql/core/src/test/resources/sql-tests/inputs/string-functions.sql

@@ -18,3 +18,9 @@ select length(uuid()), (uuid() <> uuid());
 
 -- position
 select position('bar' in 'foobarbar'), position(null, 'foobarbar'), position('aaads', null);
+
+-- left && right
+select left("abcd", 2), left("abcd", 5), left("abcd", '2'), left("abcd", null);
+select left(null, -2), left("abcd", -2), left("abcd", 0), left("abcd", 'a');
+select right("abcd", 2), right("abcd", 5), right("abcd", '2'), right("abcd", null);
+select right(null, -2), right("abcd", -2), right("abcd", 0), right("abcd", 'a');

sql/core/src/test/resources/sql-tests/results/string-functions.sql.out

@@ -1,5 +1,5 @@
 -- Automatically generated by SQLQueryTestSuite
--- Number of queries: 8
+-- Number of queries: 12
 
 
 -- !query 0
@@ -86,3 +86,35 @@ select position('bar' in 'foobarbar'), position(null, 'foobarbar'), position('aa
 struct<locate(bar, foobarbar, 1):int,locate(CAST(NULL AS STRING), foobarbar, 1):int,locate(aaads, CAST(NULL AS STRING), 1):int>
 -- !query 7 output
 4	NULL	NULL
+
+
+-- !query 8
+select left("abcd", 2), left("abcd", 5), left("abcd", '2'), left("abcd", null)
+-- !query 8 schema
+struct<left('abcd', 2):string,left('abcd', 5):string,left('abcd', '2'):string,left('abcd', NULL):string>
+-- !query 8 output
+ab	abcd	ab	NULL
+
+
+-- !query 9
+select left(null, -2), left("abcd", -2), left("abcd", 0), left("abcd", 'a')
+-- !query 9 schema
+struct<left(NULL, -2):string,left('abcd', -2):string,left('abcd', 0):string,left('abcd', 'a'):string>
+-- !query 9 output
+NULL			NULL
+
+
+-- !query 10
+select right("abcd", 2), right("abcd", 5), right("abcd", '2'), right("abcd", null)
+-- !query 10 schema
+struct<right('abcd', 2):string,right('abcd', 5):string,right('abcd', '2'):string,right('abcd', NULL):string>
+-- !query 10 output
+cd	abcd	cd	NULL
+
+
+-- !query 11
+select right(null, -2), right("abcd", -2), right("abcd", 0), right("abcd", 'a')
+-- !query 11 schema
+struct<right(NULL, -2):string,right('abcd', -2):string,right('abcd', 0):string,right('abcd', 'a'):string>
+-- !query 11 output
+NULL			NULL