MySQL auto populate number field base on another field - mysql

Question

mysql question... So I would like to auto populate a number, but I would like it to be based on another field. I'm not sure how to explain it other than using this example:
Field 1 | Field 2 (auto populate)
1 | 1
2 | 1
2 | 2
2 | 3
3 | 1
3 | 2
2 | 4
2 | 5
Is there a mysql function expression that can be added to the column?
I know I could do this in my Python script that is feeding the values, but I have to assume it will be faster if the function is baked into mysql.
UPDATE:
Let me add some clarification...
Field 1 will get filled with a value from a script.
There will be multiple entries of Field 1 that are the same number.
I would like Field 2 to auto populate to have a unique number WITHIN that Field 1 number.

Answer 1

You can just store the field1 and generate field2 as increasing integer value within each field1 using user variables like this:
select
t.field1,
#rn := if(field1 = #f1, #rn + 1, if(#fi := field1, 1, 1)) field2
from (
select
t.*
from your_table t
order by field1 --very important
) t cross join (select #rn := 0, #f1 := -1) t2

Answer 2

You could use a trigger which is a function which automatically runs on certain conditions.
Here is the manual http://dev.mysql.com/doc/refman/5.7/en/trigger-syntax.html.
You could for example use a AFTER UPDATE or AFTER INSERT trigger for this purpose.

Answer 3

Setting aside the usual caveats relating to MyISAM tables...
DROP TABLE IF EXISTS my_table;
CREATE TABLE my_table
(field1 INT NOT NULL
,field2 INT NOT NULL AUTO_INCREMENT
,PRIMARY KEY(field1,field2)
) ENGINE = MyISAM;
INSERT INTO my_table (field1) VALUES
(1),(2),(2),(2),(3),(3),(2),(2);
SELECT * FROM my_table;
+--------+--------+
| field1 | field2 |
+--------+--------+
| 1 | 1 |
| 2 | 1 |
| 2 | 2 |
| 2 | 3 |
| 2 | 4 |
| 2 | 5 |
| 3 | 1 |
| 3 | 2 |
+--------+--------+
..but I would question whether there's really any sense storing this information at all...
...hence, a more robust approach...
DROP TABLE IF EXISTS my_table;
CREATE TABLE my_table
(field1 INT NOT NULL
,field2 INT NOT NULL AUTO_INCREMENT PRIMARY KEY
) ENGINE = InnoDB;
INSERT INTO my_table (field1) VALUES
(1),(2),(2),(2),(3),(3),(2),(2);
SELECT x.field1
, CASE WHEN #prev = field1 THEN #i:=#i+1 ELSE #i:=1 END i
, #prev:=field1 prev
FROM my_table x
, (SELECT #prev:=null,#i:=0) vars
ORDER
BY field1
, field2;
+--------+------+------+
| field1 | i | prev |
+--------+------+------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 2 | 2 | 2 |
| 2 | 3 | 2 |
| 2 | 4 | 2 |
| 2 | 5 | 2 |
| 3 | 1 | 3 |
| 3 | 2 | 3 |
+--------+------+------+

Answer 4

You probably want to create a trigger. Something like the following:
CREATE TRIGGER `populate_field_2` BEFORE INSERT ON `table_name` FOR EACH ROW BEGIN
DECLARE _new_val INT DEFAULT 0;
SELECT MAX(field_2)+1 INTO _new_val FROM 'table_name' WHERE field_1 = NEW.field_1;
SET NEW.field_2 = _new_val;
END
This would populate field_2 each time you created a new row. You can update the logic to create whatever value for field2 you need.