Question

I have a query that is selecting about 20 columns, all are varchars.

Some will be empty, which is just fine. But I want to put a default string of “——-” for any columns that are empty.

Here’s what I have so far, that works:

$sql = "SELECT col1, ";
$sql .= "IF(`col2` IS NULL,'----------',`col2`) AS `col2`, ";
$sql .= "IF(`col3` IS NULL,'----------',`col3`) AS `col3`, ";
$sql .= "IF(`col4` IS NULL,'----------',`col4`) AS `col4`, ";
$sql .= "IF(`col5` IS NULL,'----------',`col5`) AS `col5`, ";
$sql .= "IF(`col6` IS NULL,'----------',`col6`) AS `col6`, ";
.......and so on.....
$sql .= "FROM `tableName`";

This works for me, but I am wondering if there’s a more “global” way, instead of specifying each column.

Answer 1

Following mfloryan advice you can easily generate your string with a loop and implode() function

<?php
$sql = 'select col1,';
$a = array();
for ($i=2;$i<=20;$i++){
$a[] = "coalesce(col".$i.",'-----') as col".$i."\r\n";
}
$sql.=implode(',',$a);
$sql.=" from table_name";
echo $sql;
?>

edit. Note that information_schema is a wonderful tool to create dynamic SQL. This is an example

mysql> use test;
Database changed
mysql> create table dynSQL(
    -> col1 varchar(10),
    -> col2 varchar(10),
    -> col3 varchar(10),
    -> col4 varchar(10)
    -> ) engine = myisam;
Query OK, 0 rows affected (0.04 sec)

mysql>
mysql> insert into dynSQL(col1,col2,col3,col4)
    -> values (1,3,null,4),(10,null,4,null),(20,4,5,null);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql>
mysql> set @str = (select concat('select col1,',group_concat(concat("coalesce(",column_name,",'----') as ",column_name,"\n")),' from dynSQL')
    -> from information_schema.columns
    -> where table_schema = 'test' and table_name = 'dynSQL' and column_name like 'col%' and column_name <> 'col1');
Query OK, 0 rows affected (0.00 sec)

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

mysql> execute stmt;
+------+------+------+------+
| col1 | col2 | col3 | col4 |
+------+------+------+------+
| 1    | 3    | ---- | 4    |
| 10   | ---- | 4    | ---- |
| 20   | 4    | 5    | ---- |
+------+------+------+------+
3 rows in set (0.00 sec)

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

Regards. Nick