MySQL: set field default value to other column

defaultdefault-valueMySQL

How to set default value for a field to other column in MySQL?

I have done it in Oracle with virtual field, but I do not know how to do it in MySQL.

This is my table:

CREATE TABLE TSM_TRANSACTION_TBL
(
  TRANS_ID     INT primary key auto_increment,
  LOCATION_ID  INT,
  TRANS_DATE   DATE,
  RESOURCE_ID  INT,
  TS_ID        INT,
  MAX_VALUE    INT,
  BOOKED_UNITS INT default 0,
  REMAINING    INT default MAX_VALUE - BOOKED_UNITS,
  BOOKED       INT not null,
  USER_ID      INT,
  TRANS_TIME   TIMESTAMP
);

Best Answer

As documented under Data Type Default Values:

The DEFAULT value clause in a data type specification indicates a default value for a column. With one exception, the default value must be a constant; it cannot be a function or an expression. This means, for example, that you cannot set the default for a date column to be the value of a function such as NOW() or CURRENT_DATE. The exception is that you can specify CURRENT_TIMESTAMP as the default for a TIMESTAMP column. See Section 11.3.5, “Automatic Initialization and Updating for TIMESTAMP”.

Instead, you can define an insertion trigger:

CREATE TRIGGER foo BEFORE INSERT ON TSM_TRANSACTION_TBL FOR EACH ROW
  IF NEW.REMAINING IS NULL THEN
    SET NEW.REMAINING := NEW.MAX_VALUE - NEW.BOOKED_UNITS;
  END IF;;

Related Solutions

Mysql – How to set a default value for a MySQL Datetime column

IMPORTANT EDIT: It is now possible to achieve this with DATETIME fields since MySQL 5.6.5, take a look at the other post below...

Previous versions can't do that with DATETIME...

But you can do it with TIMESTAMP:

mysql> create table test (str varchar(32), ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP);
Query OK, 0 rows affected (0.00 sec)

mysql> desc test;
+-------+-------------+------+-----+-------------------+-------+
| Field | Type        | Null | Key | Default           | Extra |
+-------+-------------+------+-----+-------------------+-------+
| str   | varchar(32) | YES  |     | NULL              |       | 
| ts    | timestamp   | NO   |     | CURRENT_TIMESTAMP |       | 
+-------+-------------+------+-----+-------------------+-------+
2 rows in set (0.00 sec)

mysql> insert into test (str) values ("demo");
Query OK, 1 row affected (0.00 sec)

mysql> select * from test;
+------+---------------------+
| str  | ts                  |
+------+---------------------+
| demo | 2008-10-03 22:59:52 | 
+------+---------------------+
1 row in set (0.00 sec)

mysql>

CAVEAT: IF you define a column with CURRENT_TIMESTAMP ON as default, you will need to ALWAYS specify a value for this column or the value will automatically reset itself to "now()" on update. This means that if you do not want the value to change, your UPDATE statement must contain "[your column name] = [your column name]" (or some other value) or the value will become "now()". Weird, but true. I am using 5.5.56-MariaDB

Mysql – concatenate multiple MySQL rows into one field

You can use GROUP_CONCAT:

SELECT person_id,
   GROUP_CONCAT(hobbies SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;

As Ludwig stated in his comment, you can add the DISTINCT operator to avoid duplicates:

SELECT person_id,
   GROUP_CONCAT(DISTINCT hobbies SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;

As Jan stated in their comment, you can also sort the values before imploding it using ORDER BY:

SELECT person_id, 
       GROUP_CONCAT(hobbies ORDER BY hobbies ASC SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;

As Dag stated in his comment, there is a 1024 byte limit on the result. To solve this, run this query before your query:

SET group_concat_max_len = 2048;

Of course, you can change 2048 according to your needs. To calculate and assign the value:

SET group_concat_max_len = CAST(
                     (SELECT SUM(LENGTH(hobbies)) + COUNT(*) * LENGTH(', ')
                           FROM peoples_hobbies
                           GROUP BY person_id) AS UNSIGNED);

Best Answer

Related Solutions

Mysql – How to set a default value for a MySQL Datetime column

Mysql – concatenate multiple MySQL rows into one field

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