[SPARK-34747][SQL][DOCS] Add virtual operators to the built-in function document

### What changes were proposed in this pull request?

This PR fix an issue that virtual operators (`||`, `!=`, `<>`, `between` and `case`) are absent from the Spark SQL Built-in functions document.

### Why are the changes needed?

The document should explain about all the supported built-in operators.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Built the document with `SKIP_SCALADOC=1 SKIP_RDOC=1 SKIP_PYTHONDOC=1 bundler exec jekyll build` and then, confirmed the document.

![neq1](https://user-images.githubusercontent.com/4736016/111192859-e2e76380-85fc-11eb-89c9-75916a5e856a.png)
![neq2](https://user-images.githubusercontent.com/4736016/111192874-e7ac1780-85fc-11eb-9a9b-c504265b373f.png)
![between](https://user-images.githubusercontent.com/4736016/111192898-eda1f880-85fc-11eb-992d-cf80c544ec27.png)
![case](https://user-images.githubusercontent.com/4736016/111192918-f266ac80-85fc-11eb-9306-5dbc413a0cdb.png)
![double_pipe](https://user-images.githubusercontent.com/4736016/111192952-fb577e00-85fc-11eb-932e-385e5c2a5205.png)

Closes apache#31841 from sarutak/builtin-op-doc.

Authored-by: Kousuke Saruta <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>

Author: sarutak

sql/core/src/main/scala/org/apache/spark/sql/execution/command/functions.scala

@@ -229,8 +229,10 @@ case class ShowFunctionsCommand(
           case (f, "USER") if showUserFunctions => f.unquotedString
           case (f, "SYSTEM") if showSystemFunctions => f.unquotedString
         }
-    // Hard code "<>", "!=", "between", and "case" for now as there is no corresponding functions.
-    // "<>", "!=", "between", and "case" is SystemFunctions, only show when showSystemFunctions=true
+    // Hard code "<>", "!=", "between", "case", and "||"
+    // for now as there is no corresponding functions.
+    // "<>", "!=", "between", "case", and "||" is SystemFunctions,
+    // only show when showSystemFunctions=true
     if (showSystemFunctions) {
       (functionNames ++
         StringUtils.filterPattern(FunctionsCommand.virtualOperators, pattern.getOrElse("*")))

sql/gen-sql-api-docs.py

@@ -24,6 +24,106 @@
 ExpressionInfo = namedtuple(
     "ExpressionInfo", "className name usage arguments examples note since deprecated")
 
+_virtual_operator_infos = [
+    ExpressionInfo(
+        className="",
+        name="!=",
+        usage="expr1 != expr2 - Returns true if `expr1` is not equal to `expr2`, " +
+              "or false otherwise.",
+        arguments="\n    Arguments:\n      " +
+                  """* expr1, expr2 - the two expressions must be same type or can be casted to
+                       a common type, and must be a type that can be used in equality comparison.
+                       Map type is not supported. For complex types such array/struct,
+                       the data types of fields must be orderable.""",
+        examples="\n    Examples:\n      " +
+                 "> SELECT 1 != 2;\n      " +
+                 " true\n      " +
+                 "> SELECT 1 != '2';\n      " +
+                 " true\n      " +
+                 "> SELECT true != NULL;\n      " +
+                 " NULL\n      " +
+                 "> SELECT NULL != NULL;\n      " +
+                 " NULL",
+        note="",
+        since="1.0.0",
+        deprecated=""),
+    ExpressionInfo(
+        className="",
+        name="<>",
+        usage="expr1 != expr2 - Returns true if `expr1` is not equal to `expr2`, " +
+              "or false otherwise.",
+        arguments="\n    Arguments:\n      " +
+                  """* expr1, expr2 - the two expressions must be same type or can be casted to
+                       a common type, and must be a type that can be used in equality comparison.
+                       Map type is not supported. For complex types such array/struct,
+                       the data types of fields must be orderable.""",
+        examples="\n    Examples:\n      " +
+                 "> SELECT 1 != 2;\n      " +
+                 " true\n      " +
+                 "> SELECT 1 != '2';\n      " +
+                 " true\n      " +
+                 "> SELECT true != NULL;\n      " +
+                 " NULL\n      " +
+                 "> SELECT NULL != NULL;\n      " +
+                 " NULL",
+        note="",
+        since="1.0.0",
+        deprecated=""),
+    ExpressionInfo(
+        className="",
+        name="between",
+        usage="expr1 [NOT] BETWEEN expr2 AND expr3 - " +
+              "evaluate if `expr1` is [not] in between `expr2` and `expr3`.",
+        arguments="",
+        examples="\n    Examples:\n      " +
+                 "> SELECT col1 FROM VALUES 1, 3, 5, 7 WHERE col1 BETWEEN 2 AND 5;\n      " +
+                 " 3\n      " +
+                 " 5",
+        note="",
+        since="1.0.0",
+        deprecated=""),
+    ExpressionInfo(
+        className="",
+        name="case",
+        usage="CASE expr1 WHEN expr2 THEN expr3 " +
+              "[WHEN expr4 THEN expr5]* [ELSE expr6] END - " +
+              "When `expr1` = `expr2`, returns `expr3`; " +
+              "when `expr1` = `expr4`, return `expr5`; else return `expr6`.",
+        arguments="\n    Arguments:\n      " +
+                  "* expr1 - the expression which is one operand of comparison.\n      " +
+                  "* expr2, expr4 - the expressions each of which is the other " +
+                  "  operand of comparison.\n      " +
+                  "* expr3, expr5, expr6 - the branch value expressions and else value expression" +
+                  "  should all be same type or coercible to a common type.",
+        examples="\n    Examples:\n      " +
+                 "> SELECT CASE col1 WHEN 1 THEN 'one' " +
+                 "WHEN 2 THEN 'two' ELSE '?' END FROM VALUES 1, 2, 3;\n      " +
+                 " one\n      " +
+                 " two\n      " +
+                 " ?\n      " +
+                 "> SELECT CASE col1 WHEN 1 THEN 'one' " +
+                 "WHEN 2 THEN 'two' END FROM VALUES 1, 2, 3;\n      " +
+                 " one\n      " +
+                 " two\n      " +
+                 " NULL",
+        note="",
+        since="1.0.1",
+        deprecated=""),
+    ExpressionInfo(
+        className="",
+        name="||",
+        usage="expr1 || expr2 - Returns the concatenation of `expr1` and `expr2`.",
+        arguments="",
+        examples="\n    Examples:\n      " +
+                 "> SELECT 'Spark' || 'SQL';\n      " +
+                 " SparkSQL\n      " +
+                 "> SELECT array(1, 2, 3) || array(4, 5) || array(6);\n      " +
+                 " [1,2,3,4,5,6]",
+        note="\n    || for arrays is available since 2.4.0.\n",
+        since="2.3.0",
+        deprecated="")
+]
+
 
 def _list_function_infos(jvm):
     """
@@ -32,7 +132,7 @@ def _list_function_infos(jvm):
     """
 
     jinfos = jvm.org.apache.spark.sql.api.python.PythonSQLUtils.listBuiltinFunctionInfos()
-    infos = []
+    infos = _virtual_operator_infos
     for jinfo in jinfos:
         name = jinfo.getName()
         usage = jinfo.getUsage()