PostgreSQL for MySQL Users: Unterschied zwischen den Versionen

Version vom 29. August 2017, 13:55 Uhr

Siehe auch:

• PostgreSQL

Licensing:

• MySQL: Dual Licensed
• PostgreSQL: BSD-Style

Internet Resources

• Things to find out about when moving from MySQL to PostgreSQL (PostgreSQL wiki)
• PostgreSQL for MySQL Administrators (opslife blog)
• PostgreSQL for MySQL users (coderholic blog)
• Switching from MySQL to PostgreSQL - tips, tricks and gotchas? on StackOverflow

MySQL's INSERT UPDATE

Option 1) Write a function like:

 CREATE FUNCTION doinsert(_id integer, _value text) RETURNS void AS $$
 BEGIN
  UPDATE thetable SET value = _value WHERE id = _id;
  IF NOT FOUND THEN
     INSERT INTO thetable(id, value) VALUES (_id, _value);
   END IF
 END;
 $$ LANGUAGE plpgsql;

Option 2) Use two SQL statements:

 -- update the existing
 UPDATE realTable SET VALUE = (SELECT VALUE FROM tmp WHERE tmp.id = realTable.id)
 WHERE EXISTS (SELECT VALUE FROM tmp WHERE tmp.id = realTable.id);
 -- insert the missing
 INSERT INTO realTable(id, value)
 SELECT id, value FROM tmp WHERE NOT EXISTS(SELECT 1 FROM realTable WHERE tmp.id = realTable.id);

MySQL's AUTO INCREMENT

• Oracle: SEQUENCE
• MySQL: id INT UNSIGNED NOT NULL AUTO_INCREMENT
• PostgreSQL: id SERIAL oder SEQUENCE

Syntactic sugar: SERIAL

CREATE TABLE table_name (
  Identifier SERIAL,
  -- ...
);

Explizite Variante: SEQUENCE

SEQUENCE-Definition

CREATE SEQUENCE myseq;

SEQUENCE-Verwendung

entweder als Standard-Wert der Spalte:

CREATE TABLE table_name (
  Identifier NUMBER DEFAULT nextval('myseq'),
  -- ...
);

oder explizit beim Einfügen neuer Zeilen:

INSERT ... INTO table_name VALUES(nextval('myseq'), 'Name', ...);

• Siehe auch 1 und 2

MySQL's DROP TABLE IF EXIST

• Existiert auch in PostgreSQL ab Version 8.2
• Bei früheren Versionen gibt es einen Workaround:
  •  % select count(*) from pg_class where relname = 'mytable'
  • return 1 if mytable exists, and 0 if it doesn't
  • so you can run a DROP TABLE command (or not) based on the returned value.

MySQL's INT UNSIGNED (Cardinal)

• MySQL: column INT UNSIGNED
• PostgreSQL: column INT CHECK (column > 0)

MySQL's Index-Typ

• MySQL: INDEX idx_id(id)
• PostgreSQL: CREATE INDEX idx_id ON TABLENAME(id);

MySQL's Query Resultset Limit

• MySQL: SELECT * FROM tbl LIMIT 10, 5
• PostgreSQL: SELECT * FROM tbl LIMIT 5 OFFSET 10

Data types of MySQL versus PostgreSQL