How to delete columns before a certain column in SQL [duplicate] - mysql

Question

I am trying to remove a column from a MySQL table that I am using with Handsontable. When I remove a column from the Handsontable, I can get the column's index using the afterRemoveCol() callback:
afterRemoveCol: function (index, amount) {
alert(index +' amount: '+amount);
}
I would like to remove the column using the column number (n) returned by this callback function from the MySQL table using something like:
ALTER TABLE tbl_Blah DROP COLUMN n;
So, if I want to drop column #3 from the MySQL table, How would I do this using just the column number?

Answer 1

To add to RMathis answer, you can do everything within SQL by also using SET to define the DROP string in conjuntion with PREPARE and EXECUTE
SQL Fiddle
MySQL 5.6 Schema Setup:
CREATE TABLE Table1
(`col1` varchar(1),
`col2` varchar(1),
`col3` varchar(1),
`col4` varchar(1),
`col5` varchar(1))
;
set #col = (select column_name
from information_schema.columns
where table_name='table1' and ordinal_position=3);
SET #s = CONCAT("alter table table1 drop column ", #col);
PREPARE stmt FROM #s;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
Query 1:
desc table1
Results:
| COLUMN_NAME | COLUMN_TYPE | IS_NULLABLE | COLUMN_KEY | COLUMN_DEFAULT | EXTRA |
|-------------|-------------|-------------|------------|----------------|-------|
| col1 | varchar(1) | YES | | (null) | |
| col2 | varchar(1) | YES | | (null) | |
| col4 | varchar(1) | YES | | (null) | |
| col5 | varchar(1) | YES | | (null) | |

Answer 2

SQL from SQL can generate the drop statement using ordinal_position from information_schema.columns.
mysql test> create table test.tab (col1 char(1), col2 char(1), col3 char(1), col4 char(1), col5 char(1));
Query OK, 0 rows affected (0.01 sec)
mysql test> desc test.tab;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| col1 | char(1) | YES | | NULL | |
| col2 | char(1) | YES | | NULL | |
| col3 | char(1) | YES | | NULL | |
| col4 | char(1) | YES | | NULL | |
| col5 | char(1) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
5 rows in set (0.00 sec)
This query is generating the statement to drop the third column...
mysql test> select concat('alter table ',table_schema,'.',table_name,' drop column ',column_name,';') as query1 from information_schema.columns where table_schema='test' and table_name='tab' and ordinal_position=3;
+----------------------------------------+
| query1 |
+----------------------------------------+
| alter table test.tab drop column col3; |
+----------------------------------------+
1 row in set (0.00 sec)
Now to run the projected command...
mysql test> alter table test.tab drop column col3;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
The column definition has been removed from the table.
mysql test> desc test.tab;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| col1 | char(1) | YES | | NULL | |
| col2 | char(1) | YES | | NULL | |
| col4 | char(1) | YES | | NULL | |
| col5 | char(1) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
4 rows in set (0.01 sec)

Answer 3

Name them (columns in your db mirroring the spreadsheet) with the numbers/letters in full correspondence with your spreadsheet.
And then you'd be able to use regular ALTER TABLE ... DROP COLUMN.