For example, here’s a table that has a primary key but is not AUTO_INCREMENT: mysql> CREATE TABLE foo ( id INT NOT NULL, PRIMARY KEY (id) ); mysql> INSERT INTO foo VALUES (1), (2), (5); You can MODIFY the column to redefine it with the AUTO_INCREMENT option: mysql> ALTER TABLE foo MODIFY COLUMN id INT … Read more
INSERT and UPDATE performance varies little: it will be almost same for (INT) and (INT, INT) keys. SELECT performance of composite PRIMARY KEY depends on many factors. If your table is InnoDB, then the table is implicitly clustered on the PRIMARY KEY value. That means that searches for both values will be faster if the … Read more
You’ve already had a few good answers here on how to do exactly as you ask.. For reference let me just mention the recommended way to do this in Hibernate instead, which is to use a surrogate key as primary key, and to mark business keys as NaturalId’s: Although we recommend the use of surrogate … Read more
According to The Queen Of Indexing – Kimberly Tripp – what she looks for in a clustered index is primarily: Unique Narrow Static And if you can also guarantee: Ever-increasing pattern then you’re pretty close to having your ideal clustering key! Check out her entire blog post here, and another really interesting one about clustering … Read more
… that can be used to insert values into other tables? You can even do that in a single SQL statement using a data-modifying CTE: WITH ins1 AS ( INSERT INTO tbl1(txt) VALUES (‘foo’) RETURNING tbl1_id ) INSERT INTO tbl2(tbl1_id) SELECT * FROM ins1 Requires PostgreSQL 9.1 or later. db<>fiddle here (Postgres 13) Old sqlfiddle … Read more
Well. Oracle uses sequences and default values for IDENTITY functionality in 12c. Therefore you need to know about sequences for your question. First create a test identity table. CREATE TABLE IDENTITY_TEST_TABLE ( ID NUMBER GENERATED ALWAYS AS IDENTITY , NAME VARCHAR2(30 BYTE) ); First, lets find your sequence name that is created with this identity … Read more
It is not possible. Relations in EF follows exactly same rules as in the database. It means that principal table must have unique identifier which is referenced by dependent table. In case of database the identifier can be either primary key or unique column(s) of principal table. Otherwise it is not valid relation. Entity framework … Read more
A table should have a primary key so that you could identify each row uniquely with it. Technically, you can have tables without a primary key, but you’ll be breaking good database design rules.
Let’s for the sake of simplicity assume you have two records id name ——— 1 john id name ——— 2 jim both from table t (but they can come from different tables) You could do UPDATE t, t as t2 SET t.id = t2.id, t2.id = t.id WHERE t.id = 1 AND t2.id = 2 … Read more