Add support for try_cast function on sqlalchemy.dialects.mssql

Added new :func:`.mssql.try_cast` construct for SQL Server which emits
"TRY_CAST" syntax.  Pull request courtesy Leonel Atencio.

Fixes: #4782
Closes: #4785
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4785
Pull-request-sha: cf13303a9d1c0cc0233a82a5d2ca01f438b6fb9b

Change-Id: I74a71ff5e587353f67472534aabe0d54ae8039ae

1 files changed, 10 insertions, 0 deletions

diff --git a/test/dialect/mssql/test_compiler.py b/test/dialect/mssql/test_compiler.py
index 498de763c..642cee4cd 100644
--- a/test/dialect/mssql/test_compiler.py
+++ b/test/dialect/mssql/test_compiler.py
@@ -22,6 +22,7 @@ from sqlalchemy import update
 from sqlalchemy.dialects import mssql
 from sqlalchemy.dialects.mssql import base
 from sqlalchemy.dialects.mssql import mxodbc
+from sqlalchemy.dialects.mssql.base import try_cast
 from sqlalchemy.sql import column
 from sqlalchemy.sql import quoted_name
 from sqlalchemy.sql import table
@@ -1196,6 +1197,15 @@ class CompileTest(fixtures.TestBase, AssertsCompiledSQL):
             schema.CreateIndex(idx), "CREATE INDEX foo ON test (x) INCLUDE (y)"
         )
 
+    def test_try_cast(self):
+        metadata = MetaData()
+        t1 = Table("t1", metadata, Column("id", Integer, primary_key=True))
+
+        self.assert_compile(
+            select([try_cast(t1.c.id, Integer)]),
+            "SELECT TRY_CAST (t1.id AS INTEGER) AS anon_1 FROM t1",
+        )
+
 
 class SchemaTest(fixtures.TestBase):
     def setup(self):