SQL: DROP, CREATE,… How to avoid errors?

In SQL we often want to create or drop an object (TABLE, VIEW, SEQUENCE, INDEX, SYNONYM, DATABASE, USER, SCHEMA,….)

Now we do not know if this thing exists or not. We would like to write scripts, that work in either case.

So something like

CREATE TABLE IF NOT EXISTS XYZ (...);

or

DROP TABLE IF EXISTS XYZ;
CREATE TABLE XYZ (...);

would be desirable.

This seems to be quite hard, depending on the database.

Some databases support the „IF EXISTS“ or even the slightly less useful „IF NOT EXISTS“ pattern for some of the CREATE and DROP statements:

COMMANDPostgreSQLMS-SQL-ServerOracleDB2MySQL/MariaDB
DROP DATABASE IF EXISTSxx--x
DROP FUNCTION IF EXISTS xx-xx
DROP INDEX IF EXISTSxx-xx
DROP MATERIALIZED VIEW IF EXISTSxMaterialized view not found in documentation-Materialized view not found in documentationno materialized views supported
DROP ROLE IF EXISTSxx-xx
DROP SCHEMA IF EXISTSxxno DROP SCHEMA, Oracle uses User to express the concept of a Schema? (no DROP SCHEMA found in documentation)Schema not supported.
DROP SEQUENCE IF EXISTSxx-xx
DROP TABLE IF EXISTSxx-xx
DROP TABLESPACE IF EXISTSxNot found in documentation-? (no DROP TABLESPACE found in documentation)TABLESPACE not supported
DROP TRIGGER IF EXISTSxx-xx
DROP USER IF EXISTSxx--x
DROP VIEW IF EXISTSxx-xx
CREATE OR REPLACE DATABASE----x
CREATE DATABASE IF NOT EXISTS---xx
CREATE FUNCTION IF NOT EXISTS---x-
CREATE OR REPLACE FUNCTIONxx (CREATE OR ALTER FUNCTION)x-x
CREATE OR REPLACE INDEX----x
CREATE INDEX IF NOT EXISTSx--xx
CREATE MATERIALIZED VIEW IF NOT EXISTSxMaterialized view not found in documentation-Materialized view not found in documentationno materialized views supported
CREATE OR REPLACE ROLE----x
CREATE ROLE IF NOT EXISTS---xx
CREATE SCHEMA IF NOT EXISTSx-Oracle ties the schema closely to a user. So slightly different meaning of CREATE SCHEMA...Oracle ties the schema closely to a user. So slightly different meaning of CREATE SCHEMA...Schema not supported.
CREATE OR REPLACE SEQUENCE----x
CREATE SEQUENCE IF NOT EXISTSx--xx
CREATE OR REPLACE TABLE----x
CREATE TABLE IF NOT EXISTSx--xx
CREATE TABLESPACE IF NOT EXISTS-Not found in documentation-TABLESPACE apparently not supportedTABLESPACE not supported
CREATE OR REPLACE TRIGGER-x (CREATE OR ALTER TRIGGER)x-x
CREATE TRIGGER IF NOT EXISTS---xx
CREATE OR REPLACE USER----x
CREATE USER IF NOT EXISTS----x
CREATE OR REPLACE VIEWxx (CREATE OR ALTER VIEW)x-x
CREATE VIEW IF NOT EXISTS---xx

Usually we can happily ignore the missing „IF (NOT) EXISTS“ conditions and just DROP the object and then create it. By ignoring the error message this works.

But increasingly we want to manage the database with tools that run scripts automatically and cannot distinguish as well as a DBA between „good error messages“ and „bad error messages“.

So we should find our ways around this.

The trick is to use the procedural extension language of SQL (like PL/SQL for Oracle, because in case of Oracle we need it most often). Then we write a block and find by a select in the data dictionary if the object we are creating or dropping exists. Then we put the select result into a variable and put an IF-condition around our CREATE or DROP statement. This approach can also be useful for dropping all objects of a certain type that fullfill some SQL-query-accessible condition.

DECLARE
   CNT INT;
BEGIN
   SELECT COUNT(*) INTO CNT 
     FROM USER_TABLES 
     WHERE TABLE_NAME = 'MY_TABLE';
   IF CNT >= 1 THEN
      EXECUTE IMMEDIATE 'DROP TABLE MY_TABLE';
   END IF;
END;
/

Or to catch the kind of exception that we want to ignore (example for Oracle):

BEGIN
   EXECUTE IMMEDIATE 'DROP TABLE MY_TABLE';
EXCEPTION
   WHEN OTHERS THEN
      IF SQLCODE != -942 THEN
         RAISE;
      END IF;
END;
/

It is a bit tricky to find the reference pages for the SQL dialects of the different DB products. I used these:

MariaDB is a fork of mySQL that was created, when mySQL came to Oracle. I recommend it as the successor of mySQL, if you are into mySQL-like databases. And I strongly recommend looking into PostgreSQL, if you intend to use a free/opensource database, because it is the same price and just has richer features. Generally all five databases are good and have their areas of strength and their weaknesses. I won’t go deeper into this in this article…

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Microsoft SQL Server will be available for Linux in 2017

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Microsoft has officially announced that their database MS SQL Server will become available for Linux in 2017.

I think the time has come for this. Since the departure of Steve Ballmer Microsoft has become a little bit less religous and more pragmatic. There are good reasons to be skeptical about companies like Microsoft and Oracle, but having more competition and more choice is a good thing. Maybe the database product from Oracle is slightly better than MS SQL Server, but there are very few projects where this difference really matters. So now we have five important relational DB products: DB2, Oracle, MS-SQL-Server, PostgreSQL and MariaDB (the successor of mySQL). When starting a new project with no specific constraints about the DB I would usually look at PostgreSQL first, because it is a feature rich and powerful open source database. Since database products are usually something that cannot reasonably be changed within one software system for decades this is a good thing, because we never know what the big companies want to do in such a long time scale. If the migration to another DB product is easy, then the software does not really make use of the power and the features of the DB. And it will not be easy anyway.

There are a lot of cases where the combination of MS-SQL-Server with Linux will make a lot of sense. Since there are software systems that make use of this DB product, it gives the flexibility to run the DB on Linux servers. And maybe avoid an expensive migration to another DB product. As I already said it gives one more choice. In development environments where MS-products are commonly used, it gives one more combination. And eventually it will encourage Oracle and IBM a little bit to refrain from excessive price increases.

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