Fix(duckdb): Generate correct DETACH syntax if IF EXISTS is set

sqlglot/dialects/duckdb.py

@@ -1164,3 +1164,12 @@ def numbertostr_sql(self, expression: exp.NumberToStr) -> str:
         def autoincrementcolumnconstraint_sql(self, _) -> str:
             self.unsupported("The AUTOINCREMENT column constraint is not supported by DuckDB")
             return ""
+
+        def detach_sql(self, expression: exp.Detach) -> str:
+            this = self.sql(expression, "this")
+            # the DATABASE keyword is required if IF EXISTS is set
+            # without it, DuckDB throws an error: Parser Error: syntax error at or near "exists" (Line Number: 1)
+            # ref: https://duckdb.org/docs/stable/sql/statements/attach.html#detach-syntax
+            exists_sql = " DATABASE IF EXISTS" if expression.args.get("exists") else ""
+
+            return f"DETACH{exists_sql} {this}"

tests/dialects/test_duckdb.py

@@ -1494,7 +1494,11 @@ def test_attach_detach(self):
 
         # DETACH
         self.validate_identity("DETACH new_database")
-        self.validate_identity("DETACH IF EXISTS file")
+
+        # when 'if exists' is set, the syntax is DETACH DATABASE, not DETACH
+        # ref: https://duckdb.org/docs/stable/sql/statements/attach.html#detach-syntax
+        self.validate_identity("DETACH IF EXISTS file", "DETACH DATABASE IF EXISTS file")
+        self.validate_identity("DETACH DATABASE IF EXISTS file", "DETACH DATABASE IF EXISTS file")
 
         self.validate_identity("DETACH DATABASE db", "DETACH db")