Question

Supposing a query such as:

SELECT * FROM tableA;

How can I prepend a_ to each columns’ name? For example if there is a column “username” it would be accessed in the results as “a_username”.

EDIT: The SELECT username AS a_username format will not help as I need to continue using the * field selection. There is a JOIN and a potential conflict with a returned column from another table in the JOIN. I will be iterating over the returned columns (foreach) and only want to output the columns that came from a particular table (whose schema may change) to HTML input fields where a site admin could edit the fields’ content directly. The SQL query in question looks like SELECT firstTable.*, anotherTable.someField, anotherTable.someOtherField and their exists the possibility that someField or someOtherField exists in firstTable.

Thanks.

Answer 1

You can use the INFORMATION_SCHEMA.COLUMNS table to formulate the query and then use dynamic SQL to execute it.

First let’s make a sample database called dotancohen and a table called mytable

mysql> drop database if exists dotancohen;
Query OK, 1 row affected (0.03 sec)

mysql> create database dotancohen;
Query OK, 1 row affected (0.00 sec)

mysql> use dotancohen
Database changed
mysql> create table mytable
    -> (
    ->     id int not null auto_increment,
    ->     username varchar(30),
    ->     realname varchar(30),
    ->     primary key (id)
    -> );
Query OK, 0 rows affected (0.06 sec)

mysql> insert into mytable (realname,username) values
    -> ('rolando','odnalor'),('pamela','alemap'),
    -> ('dominique','euqinimod'),('diamond','dnomaid');
Query OK, 4 rows affected (0.05 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> select * from mytable;
+----+-----------+-----------+
| id | username  | realname  |
+----+-----------+-----------+
|  1 | odnalor   | rolando   |
|  2 | alemap    | pamela    |
|  3 | euqinimod | dominique |
|  4 | dnomaid   | diamond   |
+----+-----------+-----------+
4 rows in set (0.00 sec)

mysql>

Here is the metadata table called INFORMATION_SCHEMA.COLUMNS:

mysql> desc INFORMATION_SCHEMA.COLUMNS;
+--------------------------+---------------------+------+-----+---------+-------+
| Field                    | Type                | Null | Key | Default | Extra |
+--------------------------+---------------------+------+-----+---------+-------+
| TABLE_CATALOG            | varchar(512)        | NO   |     |         |       |
| TABLE_SCHEMA             | varchar(64)         | NO   |     |         |       |
| TABLE_NAME               | varchar(64)         | NO   |     |         |       |
| COLUMN_NAME              | varchar(64)         | NO   |     |         |       |
| ORDINAL_POSITION         | bigint(21) unsigned | NO   |     | 0       |       |
| COLUMN_DEFAULT           | longtext            | YES  |     | NULL    |       |
| IS_NULLABLE              | varchar(3)          | NO   |     |         |       |
| DATA_TYPE                | varchar(64)         | NO   |     |         |       |
| CHARACTER_MAXIMUM_LENGTH | bigint(21) unsigned | YES  |     | NULL    |       |
| CHARACTER_OCTET_LENGTH   | bigint(21) unsigned | YES  |     | NULL    |       |
| NUMERIC_PRECISION        | bigint(21) unsigned | YES  |     | NULL    |       |
| NUMERIC_SCALE            | bigint(21) unsigned | YES  |     | NULL    |       |
| CHARACTER_SET_NAME       | varchar(32)         | YES  |     | NULL    |       |
| COLLATION_NAME           | varchar(32)         | YES  |     | NULL    |       |
| COLUMN_TYPE              | longtext            | NO   |     | NULL    |       |
| COLUMN_KEY               | varchar(3)          | NO   |     |         |       |
| EXTRA                    | varchar(27)         | NO   |     |         |       |
| PRIVILEGES               | varchar(80)         | NO   |     |         |       |
| COLUMN_COMMENT           | varchar(1024)       | NO   |     |         |       |
+--------------------------+---------------------+------+-----+---------+-------+
19 rows in set (0.02 sec)

mysql>

What you need from this table are the following columns:

• table_schema
• table_name
• column_name
• ordinal_position

What you are asking for is to have the column_name and the column_name prepended with a_

Here is the query and how to execute it:

select concat('select ',column_list,' from ',dbtb) into @newsql
from (select group_concat(concat(column_name,' a_',column_name)) column_list,
concat(table_schema,'.',table_name) dbtb from information_schema.columns
where table_schema = 'dotancohen' and table_name = 'mytable'
order by ordinal_position) A;
select @newsql;
prepare stmt from @newsql;
execute stmt;
deallocate prepare stmt;

Let’s execute it

mysql> select concat('select ',column_list,' from ',dbtb) into @newsql
    -> from (select group_concat(concat(column_name,' a_',column_name)) column_list,
    -> concat(table_schema,'.',table_name) dbtb from information_schema.columns
    -> where table_schema = 'dotancohen' and table_name = 'mytable'
    -> order by ordinal_position) A;
Query OK, 1 row affected (0.01 sec)

mysql> select @newsql;
+--------------------------------------------------------------------------------+
| @newsql                                                                        |
+--------------------------------------------------------------------------------+
| select id a_id,username a_username,realname a_realname from dotancohen.mytable |
+--------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> prepare stmt from @newsql;
Query OK, 0 rows affected (0.00 sec)
Statement prepared

mysql> execute stmt;
+------+------------+------------+
| a_id | a_username | a_realname |
+------+------------+------------+
|    1 | odnalor    | rolando    |
|    2 | alemap     | pamela     |
|    3 | euqinimod  | dominique  |
|    4 | dnomaid    | diamond    |
+------+------------+------------+
4 rows in set (0.01 sec)

mysql> deallocate prepare stmt;
Query OK, 0 rows affected (0.00 sec)

mysql>

Give it a Try !!!

You mentioned in your question : The SELECT username AS a_username format will not help as I need to continue using the * field selection.

All you have to do to implement my suggestion is run the query using tableA as follows:

select concat('select ',column_list,' from ',dbtb) into @newsql
from (select group_concat(concat(column_name,' a_',column_name)) column_list,
concat(table_schema,'.',table_name) dbtb from information_schema.columns
where table_schema = DATABASE() and table_name = 'tableA'
order by ordinal_position) A;

When you retrieve that query result, just use it as the query to submit to mysql_query.