As you can easily understand, in fact, if you have done the first action, you really need to be sure that in-scope data doesn’t change before you can do the second action, otherwise the result may be wrong or inconsistent. The moment you have two actions, where one depends on another, you need to make sure that the data used by both doesn’t change in the meantime because of some other action done by someone else. The problem, in fact, lies in the algorithm itself. This approach has a flaw, whatever the database you are using and no matter the database if relational or not. Many developers will solve it by trying to execute two steps: Generalizing the problem, it can be described as the requirement of insert some data into a table only if that data is not there already. Note that MySQL requires an unique or primary key constraint on AUTO_INCREMENT columns.įor more information, see Generating IDs in MySQL.Recently I found a quite common request on StackOverflow. Name VARCHAR (90 ) ) AUTO_INCREMENT = 1 - start value There is the table option AUTO_INCREMENT that allows you to define the start value, but you cannot define the increment, it is always 1: MySQL supports AUTO_INCREMENT column option that allows you to automatically generate IDs. SQL Server supports IDENTITY property and allows you to specify the initial and increment values: Note that a trigger is required as Oracle does not allow using NEXTVAL in DEFAULT clause for a column. ![]() Oracle does not support SERIAL (auto-increment, identity) columns, but this functionality can be implemented using a sequence and a trigger:ĬREATE SEQUENCE teams_id_seq START WITH 1 INCREMENT BY 1 If you remove rows from a table, you can insert removed IDs explicitly, it will not have any effect on the sequence generator. Continue, now it will use ID 9 INSERT INTO teams (name ) VALUES ( 'Newcastle United' ) ERROR: duplicate key value violates unique constraint "teams_id_key" - DETAIL: Key (id)=(8) already exists. Will try to assign ID 8 that already inserted INSERT INTO teams (name ) VALUES ( 'Some team' ) You can get an error if there is an UNIQUE constraint, or duplicate IDs can be inserted: Note that the sequence generator may have conflicts with IDs that were already inserted using explicit values. Continue using ID generator INSERT INTO teams (name ) VALUES ( 'Liverpool' ) - ID 7 is assigned Insert ID 8 explicitly INSERT INTO teams VALUES ( 8, 'Everton' ) If you insert an ID value explicitly, it has no effect on the sequence generator, and its next value remains unchanged and will be used when you insert subsequent rows: SERIAL - Specify Initial Value and Increment ERROR: null value in column "id" violates not-null constraint INSERT INTO teams VALUES ( 0, 'Reserved' ) INSERT INTO teams VALUES ( NULL, 'Some team' ) In MySQL these 2 values force ID generation, but this is not applied to PostgerSQL: ![]() Note that you cannot insert NULL, but can insert 0. Specify DEFAULT INSERT INTO teams VALUES ( DEFAULT, 'Manchester City' ) Omit serial column INSERT INTO teams (name ) VALUES ( 'Aston Villa' ) ![]() To generate a ID value, you can omit the SERIAL column in INSERT statement, or specify DEFAULT keyword: If you need a SERIAL column to be unique, you have to specify UNIQUE or PRIMARY KEY explicitly. Id INT NOT NULL DEFAULT NEXTVAL ( 'teams_id_seq' ) , is equivalent to CREATE SEQUENCE teams_id_seq When you define a SERIAL column, PostgreSQL automatically changes column to NOT NULL, creates a sequence tablename_serialcol_seq and DEFAULT NEXTVAL to select ID values from the sequence only if they are not supplied in INSERT statement:
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