From: Mike Bayer <mike_mp@zzzcomputing.com>
Date: Tue, 28 Jul 2020 20:46:53 +0000 (-0400)
Subject: Consider default FROM DUAL for MySQL
X-Git-Tag: rel_1_3_19~17^2
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=e41998981c0d1d539091bfd8a30c1f7132237a3d;p=thirdparty%2Fsqlalchemy%2Fsqlalchemy.git

Consider default FROM DUAL for MySQL

MySQL claims it doesn't require FROM DUAL for no
FROM clause even though the issue at #5481 locates
a case which requires one.  See if FROM DUAL the same
way as Oracle without attempting to guess is potentially
feasible.

Fixes: #5481
Change-Id: I2a28876c10a8ce2d121cd344dcdd837db321d4ab
(cherry picked from commit 59b65b3008de5d6c5e4318846d04e50e5851e957)
---

diff --git a/doc/build/changelog/unreleased_13/5481.rst b/doc/build/changelog/unreleased_13/5481.rst
new file mode 100644
index 0000000000..2ebac73bf7
--- /dev/null
+++ b/doc/build/changelog/unreleased_13/5481.rst
@@ -0,0 +1,8 @@
+.. change::
+    :tags: usecase, mysql
+    :tickets: 5481
+
+    The MySQL dialect will render FROM DUAL for a SELECT statement that has no
+    FROM clause but has a WHERE clause. This allows things like "SELECT 1 WHERE
+    EXISTS (subuqery)" kinds of queries to be used as well as other use cases.
+
diff --git a/lib/sqlalchemy/dialects/mysql/base.py b/lib/sqlalchemy/dialects/mysql/base.py
index d84e3770f4..87d35587aa 100644
--- a/lib/sqlalchemy/dialects/mysql/base.py
+++ b/lib/sqlalchemy/dialects/mysql/base.py
@@ -1303,6 +1303,18 @@ class MySQLCompiler(compiler.SQLCompiler):
     extract_map = compiler.SQLCompiler.extract_map.copy()
     extract_map.update({"milliseconds": "millisecond"})
 
+    def default_from(self):
+        """Called when a ``SELECT`` statement has no froms,
+        and no ``FROM`` clause is to be appended.
+
+        """
+        if self.stack:
+            stmt = self.stack[-1]["selectable"]
+            if stmt._whereclause is not None:
+                return " FROM DUAL"
+
+        return ""
+
     def visit_random_func(self, fn, **kw):
         return "rand%s" % self.function_argspec(fn)
 
diff --git a/lib/sqlalchemy/testing/suite/test_select.py b/lib/sqlalchemy/testing/suite/test_select.py
index d29449a519..23e66a3fae 100644
--- a/lib/sqlalchemy/testing/suite/test_select.py
+++ b/lib/sqlalchemy/testing/suite/test_select.py
@@ -7,9 +7,11 @@ from ..schema import Table
 from ... import bindparam
 from ... import case
 from ... import Computed
+from ... import exists
 from ... import false
 from ... import func
 from ... import Integer
+from ... import literal
 from ... import literal_column
 from ... import null
 from ... import select
@@ -703,6 +705,53 @@ class ComputedColumnTest(fixtures.TablesTest):
             eq_(res, [(100, 40), (1764, 168)])
 
 
+class ExistsTest(fixtures.TablesTest):
+    __backend__ = True
+
+    @classmethod
+    def define_tables(cls, metadata):
+        Table(
+            "stuff",
+            metadata,
+            Column("id", Integer, primary_key=True),
+            Column("data", String(50)),
+        )
+
+    @classmethod
+    def insert_data(cls, connection):
+        connection.execute(
+            cls.tables.stuff.insert(),
+            [
+                {"id": 1, "data": "some data"},
+                {"id": 2, "data": "some data"},
+                {"id": 3, "data": "some data"},
+                {"id": 4, "data": "some other data"},
+            ],
+        )
+
+    def test_select_exists(self, connection):
+        stuff = self.tables.stuff
+        eq_(
+            connection.execute(
+                select([literal(1)]).where(
+                    exists().where(stuff.c.data == "some data")
+                )
+            ).fetchall(),
+            [(1,)],
+        )
+
+    def test_select_exists_false(self, connection):
+        stuff = self.tables.stuff
+        eq_(
+            connection.execute(
+                select([literal(1)]).where(
+                    exists().where(stuff.c.data == "no data")
+                )
+            ).fetchall(),
+            [],
+        )
+
+
 class IsOrIsNotDistinctFromTest(fixtures.TablesTest):
     __backend__ = True
     __requires__ = ("supports_is_distinct_from",)