[CALCITE-5548] Add MSSQL-style CONVERT function (enabled in MSSql lib…

…rary)

Microsoft SQL Server's `CONVERT(type, exp [, style])`
function is equivalent to `CAST(exp AS type)` and the JDBC
standard function `{fn CONVERT(value, type)}`.

This function is not to be confused with standard SQL's
`CONVERT` function, which converts a character string from
one character set to another.

Close apache#3100

core/src/main/java/org/apache/calcite/sql/fun/SqlLibraryOperators.java

@@ -41,6 +41,7 @@
 import org.apache.calcite.sql.type.SqlTypeFamily;
 import org.apache.calcite.sql.type.SqlTypeName;
 import org.apache.calcite.sql.type.SqlTypeTransforms;
+import org.apache.calcite.sql.validate.SqlValidator;
 import org.apache.calcite.util.Litmus;
 import org.apache.calcite.util.Optionality;
 
@@ -142,26 +143,31 @@ private SqlLibraryOperators() {
   @LibraryOperator(libraries = {MSSQL})
   public static final SqlFunction MSSQL_CONVERT =
       SqlBasicFunction.create(SqlKind.CAST,
-              ReturnTypes.andThen(opBinding -> {
-                    // Guaranteed to be a SqlCallBinding, with 2 or 3 arguments
-                    final SqlCallBinding binding = (SqlCallBinding) opBinding;
-                    final SqlCall call = binding.getCall();
-                    return new SqlCallBinding(binding.getValidator(),
-                        binding.getScope(),
-                        SqlStdOperatorTable.CAST.createCall(
-                            call.getParserPosition(), call.operand(1),
-                            call.operand(0)));
-                  },
+              ReturnTypes.andThen(SqlLibraryOperators::transformConvert,
                   SqlCastFunction::inferReturnTypeImpl),
               OperandTypes.repeat(SqlOperandCountRanges.between(2, 3),
                   OperandTypes.ANY))
           .withName("CONVERT")
           .withFunctionType(SqlFunctionCategory.SYSTEM)
           .withOperandTypeInference(InferTypes.FIRST_KNOWN)
           .withOperandHandler(
-              OperandHandlers.of((validator, call) ->
-                  SqlStdOperatorTable.CAST.createCall(call.getParserPosition(),
-                      call.operand(1), call.operand(0))));
+              OperandHandlers.of(SqlLibraryOperators::transformConvert));
+
+  /** Transforms a call binding of {@code CONVERT} to an equivalent binding for
+   *  {@code CAST}. */
+  private static SqlCallBinding transformConvert(SqlOperatorBinding opBinding) {
+    // Guaranteed to be a SqlCallBinding, with 2 or 3 arguments
+    final SqlCallBinding binding = (SqlCallBinding) opBinding;
+    return new SqlCallBinding(binding.getValidator(), binding.getScope(),
+        transformConvert(binding.getValidator(), binding.getCall()));
+  }
+
+  /** Transforms a call to {@code CONVERT} to an equivalent call to
+   *  {@code CAST}. */
+  private static SqlCall transformConvert(SqlValidator validator, SqlCall call) {
+    return SqlStdOperatorTable.CAST.createCall(call.getParserPosition(),
+        call.operand(1), call.operand(0));
+  }
 
   /** The "DATE_PART(timeUnit, datetime)" function
    * (Databricks, Postgres, Redshift, Snowflake). */

site/_docs/reference.md

@@ -2635,8 +2635,8 @@ BigQuery's type system uses confusingly different names for types and functions:
 | b o | COSH(numeric)                                | Returns the hyperbolic cosine of *numeric*
 | o | CONCAT(string, string)                         | Concatenates two strings
 | b m p | CONCAT(string [, string ]*)                | Concatenates two or more strings
-| m | COMPRESS(string)                               | Compresses a string using zlib compression and returns the result as a binary string.
-| q | CONVERT(data_type[ ( length ) ] , expression [ , style ]) | In MSSQL, a function very similar to `CAST` with optional style operand. In Calcite implementation, however, it's simply a wrapper around CAST which parses, but ultimately ignores "style" argument.
+| m | COMPRESS(string)                               | Compresses a string using zlib compression and returns the result as a binary string
+| q | CONVERT(type, expression [ , style ])          | Equivalent to `CAST(expression AS type)`; ignores the *style* operand
 | p | CONVERT_TIMEZONE(tz1, tz2, datetime)           | Converts the timezone of *datetime* from *tz1* to *tz2*
 | b | CURRENT_DATETIME([ timeZone ])                 | Returns the current time as a TIMESTAMP from *timezone*
 | m | DAYNAME(datetime)                              | Returns the name, in the connection's locale, of the weekday in *datetime*; for example, it returns '星期日' for both DATE '2020-02-10' and TIMESTAMP '2020-02-10 10:10:10'