OpenCart DB query issue (syntax misunderstand)

Question

OpenCart v1.5.3.1, Export plugin:

    $query  = "SELECT pd.*, cg.name FROM `".DB_PREFIX."product_discount` pd ";
    $query .= "LEFT JOIN `".DB_PREFIX."customer_group` cg ON cg.customer_group_id=pd.customer_group_id ";
    $query .= "ORDER BY pd.product_id, cg.name";

Can anybody explain, what pd and cg means here?

Meet similar sintax in other places, but not sure what it means and how to work with it...

Looks like it is some common thing, but I am quite new to work with data bases yet, please help :-(

This query generate error:

Notice: Error: Unknown column 'cg.name' in 'field list' Error No: 1054

Answer 1

pd is the alias name for table `product_discount`
cg is the alias name for table `customer_group`

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Notice: Error: Unknown column 'cg.name' in 'field list' Error No: 1054

This would actually mean there is no field of name in the table customer_group