what syntax to use to update a SET column in mysql? - mysql

Question

I created a column called oilcompany that has SET data (Hunt, Pioneer, Chevron, BP)
I can enter any one of those into the oilcompany column and change from one to another one but I can not figure out how to change from one oilcompany to multiple oilcompany (eg. Hunt and BP)... any suggestion?

Answer 1

In the MySQL documentation there are not examples for UPDATE statements, but I normally use two ways to update these kind of columns:
Using text values
Using numeric values
Creating the test environment
mysql> CREATE TABLE tmp_table(
-> id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
-> oilcompany SET('Hunt', 'Pioneer', 'Chevron', 'BP')
-> );
Query OK, 0 rows affected (0.54 sec)
mysql> INSERT INTO tmp_table(oilcompany) VALUES ('Hunt'), ('Pioneer');
Query OK, 2 rows affected (0.11 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> SELECT * FROM tmp_table;
+----+------------+
| id | oilcompany |
+----+------------+
| 1 | Hunt |
| 2 | Pioneer |
+----+------------+
2 rows in set (0.00 sec)
Alternative#1: Using Text Values
As a SET is a collection of ENUM elements, and any ENUM element can be treated as a string, then we can do things like:
mysql> UPDATE tmp_table
-> SET oilcompany = 'Hunt,BP'
-> WHERE id = 1;
Query OK, 1 row affected (0.07 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> SELECT * FROM tmp_table;
+----+------------+
| id | oilcompany |
+----+------------+
| 1 | Hunt,BP |
| 2 | Pioneer |
+----+------------+
2 rows in set (0.00 sec)
Alternative#2: Using Numeric Values
Any SET element is stored internally as a 64bit number containing the combination of the bits that represent each SET element.
In our table: 'Hunt'=1, 'Pioneer'=2, 'Chevron'=4, 'BP'=8.
Also, mysql allows to use these numbers instead of text values. If we need to see the numeric value in the select, we need to use the SET column inside a numeric expression (E.g. adding zero).
Let's see the current values:
mysql> SELECT id, oilcompany+0, oilcompany FROM tmp_table;
+----+--------------+------------+
| id | oilcompany+0 | oilcompany |
+----+--------------+------------+
| 1 | 9 | Hunt,BP |
| 2 | 2 | Pioneer |
+----+--------------+------------+
2 rows in set (0.00 sec)
Here 9 = 'Hunt' (1) + 'BP' (8) and 2 = 'Pioneer' (2).
Now, let's change the Pioneer to 'Hunt' (1) + 'Chevron' (4):
mysql> UPDATE tmp_table
-> SET oilcompany = 5
-> WHERE id = 2;
Query OK, 1 row affected (0.08 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> SELECT id, oilcompany+0, oilcompany FROM tmp_table;
+----+--------------+--------------+
| id | oilcompany+0 | oilcompany |
+----+--------------+--------------+
| 1 | 9 | Hunt,BP |
| 2 | 5 | Hunt,Chevron |
+----+--------------+--------------+
2 rows in set (0.00 sec)