Use the LAST_INSERT_ID() function:
What if someone else inserts before I select my id?LAST_INSERT_ID() is connection specific, so there is no problem from race conditions.
How do I get the next value to be inserted?You don't. Insert, then find out what you did with LAST_INSERT_ID().
How do I change what number auto_increment starts with?Run a query, ALTER TABLE yourTable AUTO_INCREMENT = x; . The next insert will contain x or MAX(autoField) + 1, whichever is higher.
As an alternative, run INSERT INTO yourTable (autoField) VALUES (x); . The next insert will contain x+1 or MAX(autoField) + 1, whichever is higher.
Issuing TRUNCATE TABLE will delete all the rows in the table, and will reset the auto_increment value to 0 in most cases.
How do I renumber rows once I deleted some in the middle?Typically, you don't want to. Gaps are hardly ever a problem; if your application can't handle gaps in the sequence, you probably should rethink your application.
Can I do group-wise auto_increment?Yes, if you use the MyISAM engine.
How do I get the AUTO_INCREMENT value in a BEFORE INSERT trigger?This isn't possible. It's only available after insert.
How do I assign two fields the same auto_increment value in one query?You can't, not even with an AFTER INSERT trigger. Insert, then go back and update using LAST_INSERT_ID(). Those two statements could be wrapped into one stored procedure if you wish.
However, you can mimic this behavior with a BEFORE INSERT trigger and a second table to store the sequence position:
CREATE TABLE sequence (table_name VARCHAR(255), position INT UNSIGNED);
INSERT INTO sequence VALUES ('testTable', 0);
CREATE TABLE testTable (firstAuto INT UNSIGNED, secondAuto INT UNSIGNED);
DELIMITER //
CREATE TRIGGER testTable_BI BEFORE INSERT ON testTable FOR EACH ROW BEGIN
UPDATE sequence SET position = LAST_INSERT_ID(position + 1) WHERE table_name = 'testTable';
SET NEW.firstAuto = LAST_INSERT_ID();
SET NEW.secondAuto = LAST_INSERT_ID();
END//
DELIMITER ;
INSERT INTO testTable VALUES (NULL, NULL), (NULL, NULL);
SELECT * FROM testTable;
+-----------+------------+
| firstAuto | secondAuto |
+-----------+------------+
| 1 | 1 |
| 2 | 2 |
+-----------+------------+The same sequence table can maintain separate sequences for multiple tables (or separate sequences for different fields in the same table) by adding extra rows.
Does the auto_increment field have to be primary key?No, it only has to be indexed. It doesn't even have to be unique.
How does InnoDB handle AUTO_INCREMENT?See AUTO_INCREMENT handling in InnoDB.
General Information To ReadThere can be only one AUTO_INCREMENT column per table, it must be indexed, and it cannot have a DEFAULT value. An AUTO_INCREMENT column works properly only if it contains only positive values. Inserting a negative number is regarded as inserting a very large positive number. This is done to avoid precision problems when numbers wrap over from positive to negative and also to ensure that you do not accidentally get an AUTO_INCREMENT column that contains 0.
CREATE TABLE autoinc_test (
h INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
m INT UNSIGNED
) AUTO_INCREMENT = 100;
INSERT INTO autoinc_test ( m ) VALUES ( 1 );
SELECT * FROM autoinc_test;
+-----+------+
| h | m |
+-----+------+
| 100 | 1 |
+-----+------+The initial version of this article was copied, with permission, from Autoincrement_FAQ on 2012-10-05.
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