insert into table select max(column_name)+1 - mysql

Question

I have this mysql table built like this:
CREATE TABLE `posts` (
`post_id` INT(10) NOT NULL AUTO_INCREMENT,
`post_user_id` INT(10) NOT NULL DEFAULT '0',
`gen_id` INT(10) NOT NULL DEFAULT '0',
PRIMARY KEY (`post_user_id`, `post_id`)
)
COLLATE='utf8_general_ci'
ENGINE=MyISAM;
When I do:
insert into posts (post_user_id) values (1);
insert into posts (post_user_id) values (1);
insert into posts (post_user_id) values (2);
insert into posts (post_user_id) values (1);
select * from posts;
I get:
post_id | post_user_id | gen_id
1 1 0
2 1 0
1 2 0
3 1 0
A unique post_id is generated for each unique user.
I need the gen_id column to be 1 2 3 4 5 6 etc. How can I increment this column when I do an insert. I tried the one below, but it won't work. What's the right way to do this?
insert into posts (post_user_id,gen_id) values (1,select max(gen_id)+1 from posts);
//Select the highest gen_id and add 1 to it.

Answer 1

Try this:
INSERT INTO posts (post_user_id,gen_id)
SELECT 1, MAX(gen_id)+1 FROM posts;

Answer 2

Use a TRIGGER on your table. This sample code can get you started:
DELIMITER //
CREATE TRIGGER ai_trigger_name AFTER INSERT ON posts
FOR EACH ROW
BEGIN
UPDATE posts
SET gen_id = (SELECT MAX(gen_id) FROM posts) + 1
WHERE post_id = LAST_INSERT_ID()
LIMIT 1;
END;//
DELIMITER ;

Answer 3

For my case the first number to increment was null. I resolve with
IFNULL(MAX(number), 0) + 1
or better the query became
SELECT IFNULL(MAX(number), 0) + 1 FROM mytable;

Answer 4

Here is the table "Autos" and the data that it contains to begin with:
AutoID | Year | Make | Model | Color |Seq
1 | 2012 | Jeep |Liberty| Black | 1
2 | 2013 | BMW | 330XI | Blue | 2
The AutoID column is an auto incrementing column so it is not necessary to include it in the insert statement.
The rest of the columns are varchars except for the Seq column which is an integer column/field.
If you want to make it so that when you insert the next row into the table and the Seq column auto increments to the # 3 you need to write your query as follows:
INSERT INTO Autos
(
Seq,
Year,
Make,
Model,
Color,
)
Values
(
(SELECT MAX(Seq) FROM Autos) + 1, --this increments the Seq column
2013,'Mercedes','S550','Black');
The reason that I put the Seq column first is to ensure that it will work correctly... it does not matter where you put it, but better safe than sorry.
The Seq column should now have a value of 3 along with the added values for the rest of that row in the database.

Answer 5

The way that I intended that to be displayed did not happen...so I will start from the beginning: First I created a table.
create table Cars (
AutoID int identity (1,1) Primary Key,
Year int,
Make varchar (25),
Model varchar (25),
TrimLevel varchar (30),
Color varchar (30),
CreatedDate date,
Seq int
)
Secondly I inserted some dummy values
insert into Cars values (
2013,'Ford' ,'Explorer','XLT','Brown',GETDATE(),1),
(2011,'Hyundai' ,'Sante Fe','SE','White',GETDATE(),2),
(2009,'Jeep' ,'Liberty','Jet','Blue',GETDATE(),3),
(2005,'BMW' ,'325','','Green',GETDATE(),4),
(2008,'Chevy' ,'HHR','SS','Red',GETDATE(),5);
When the insertion is complete you should have 5 rows of data.
Since the Seq column is not an auto increment column and you want to ensure that the next Seq's row of data is automatically incremented to the # 6 and its subsequent rows are incremented as well you would need to write the following code:
INSERT INTO Cars
(
Seq,
Year,
color,
Make,
Model,
TrimLevel,
CreatedDate
)
Values
(
(SELECT MAX(Seq) FROM Cars) + 1,
2013,'Black','Mercedes','A550','AMG',GETDATE());
I have run this insert statement many times using different data just to make sure that it works correctly....hopefully this helps!