Make if_exists and if_not_exists flags on ddl statements match compiler

Added ``if_exists`` and ``if_not_exists`` parameters for all "Create" /
"Drop" constructs including :class:`.CreateSequence`,
:class:`.DropSequence`, :class:`.CreateIndex`, :class:`.DropIndex`, etc.
allowing generic "IF EXISTS" / "IF NOT EXISTS" phrases to be rendered
within DDL. Pull request courtesy Jesse Bakker.

Fixes: #7354
Closes: #8492
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/8492
Pull-request-sha: d107c6ce553bd430111607815f5b3938ffc4770c

Change-Id: I367e57b2d9216f5180bcc44e86ca6f3dc794e5ca

1 files changed, 14 insertions, 0 deletions

diff --git a/test/sql/test_constraints.py b/test/sql/test_constraints.py
index 462667bed..b1b731d66 100644
--- a/test/sql/test_constraints.py
+++ b/test/sql/test_constraints.py
@@ -765,6 +765,14 @@ class ConstraintCompilationTest(fixtures.TestBase, AssertsCompiledSQL):
         i = Index("xyz", t.c.x)
         self.assert_compile(schema.CreateIndex(i), "CREATE INDEX xyz ON t (x)")
 
+    def test_create_index_if_not_exists(self):
+        t = Table("t", MetaData(), Column("x", Integer))
+        i = Index("xyz", t.c.x)
+        self.assert_compile(
+            schema.CreateIndex(i, if_not_exists=True),
+            "CREATE INDEX IF NOT EXISTS xyz ON t (x)",
+        )
+
     def test_drop_index_plain_unattached(self):
         self.assert_compile(
             schema.DropIndex(Index(name="xyz")), "DROP INDEX xyz"
@@ -775,6 +783,12 @@ class ConstraintCompilationTest(fixtures.TestBase, AssertsCompiledSQL):
             schema.DropIndex(Index(name="xyz")), "DROP INDEX xyz"
         )
 
+    def test_drop_index_if_exists(self):
+        self.assert_compile(
+            schema.DropIndex(Index(name="xyz"), if_exists=True),
+            "DROP INDEX IF EXISTS xyz",
+        )
+
     def test_create_index_schema(self):
         t = Table("t", MetaData(), Column("x", Integer), schema="foo")
         i = Index("xyz", t.c.x)