In a join, how to prefix all column names with the table it came from

Question

I\'m analysing a rather horrible legacy database/codebase, trying to reduce server load by combining queries into joins (including an email alert cron job that typically inv

Answer 1

The way to dynamically name columns is to generate a prepared statement that references the information_schema. This would give you the results you were looking for.

SET @sql = NULL;
SELECT CONCAT(
   'SELECT ',GROUP_CONCAT(c.TABLE_NAME,'.',c.COLUMN_NAME,' AS `',c.TABLE_NAME,'.',c.COLUMN_NAME,'`'),'
    FROM class_alerts_holding 
    INNER JOIN class_listings ON class_listings.id = class_alerts_holding.lid 
    INNER JOIN class_users ON class_users.id = class_alerts_holding.uid
    LEFT JOIN class_prodimages ON class_prodimages.pid = class_alerts_holding.lid'
)
INTO @sql
FROM INFORMATION_SCHEMA.COLUMNS c
WHERE c.TABLE_NAME IN ('class_alerts_holding','class_listings',
                       'class_users','class_prodimages');    
PREPARE sql_statement FROM @sql;
EXECUTE sql_statement;

The GROUP_CONCAT() function has a default limit of 1024 characters, so depending on the number of columns in your tables, you may need to raise this limit in order to generate the prepared statement.

SET SESSION group_concat_max_len = 1000000;

This command will raise the group concat limit if needed. -

Answer 2

You may try dynamic sql to create a query on the go as per the table definition.

declare @col varchar(max)
set @col = Select stuff( 
          (select ', ' + column_name + '.' + table_name 
           from information_schema.columns 
           where table_name in ( 'table1', 'table2' ...) for xml 
           path('')),1,1,'')

declare @query nvarchar(max) = '
select ' + @col + ' 
from table1 
inner join table2 on table1.id = table2.id '

exec sp_executesql @query

Answer 3

You could

select ah.*, l.*, u.*, pi.* from ...

then the columns will be returned ordered by table at least.

For better distinction between every two sets of columns, you could also add "delimiter" columns like this:

select ah.*, ':', l.*, ':', u.*, ':', pi.* from ...

^{(Edited to remove explicit aliases as unnecessary, see comments.)}