More tests for the simple_* methods. (matrix-org#16596)

Expand tests for the simple_* database methods, additionally
test against both PostgreSQL and SQLite variants.

changelog.d/16596.misc

@@ -0,0 +1 @@
+Improve tests of the SQL generator.

synapse/storage/database.py

@@ -1401,12 +1401,12 @@ def simple_upsert_txn_native_upsert(
             allvalues.update(values)
             latter = "UPDATE SET " + ", ".join(k + "=EXCLUDED." + k for k in values)
 
-        sql = "INSERT INTO %s (%s) VALUES (%s) ON CONFLICT (%s) %s DO %s" % (
+        sql = "INSERT INTO %s (%s) VALUES (%s) ON CONFLICT (%s) %sDO %s" % (
             table,
             ", ".join(k for k in allvalues),
             ", ".join("?" for _ in allvalues),
             ", ".join(k for k in keyvalues),
-            f"WHERE {where_clause}" if where_clause else "",
+            f"WHERE {where_clause} " if where_clause else "",
             latter,
         )
         txn.execute(sql, list(allvalues.values()))
@@ -2062,9 +2062,7 @@ def simple_update_many_txn(
             where_clause = ""
 
         # UPDATE mytable SET col1 = ?, col2 = ? WHERE col3 = ? AND col4 = ?
-        sql = f"""
-            UPDATE {table} SET {set_clause} {where_clause}
-        """
+        sql = f"UPDATE {table} SET {set_clause} {where_clause}"
 
         txn.execute_batch(sql, args)
 
@@ -2283,17 +2281,14 @@ def simple_delete_many_txn(
         if not values:
             return 0
 
-        sql = "DELETE FROM %s" % table
-
         clause, values = make_in_list_sql_clause(txn.database_engine, column, values)
         clauses = [clause]
 
         for key, value in keyvalues.items():
             clauses.append("%s = ?" % (key,))
             values.append(value)
 
-        if clauses:
-            sql = "%s WHERE %s" % (sql, " AND ".join(clauses))
+        sql = "DELETE FROM %s WHERE %s" % (table, " AND ".join(clauses))
         txn.execute(sql, values)
 
         return txn.rowcount