The moment the left join becomes clear
Mastering the LEFT join
The left join is very powerful, very useful and very under used. In my experience few people have even heard of the left join, let alone used one. The left join comes in handy when you are trying to join tables together where you have unreliable joins or where you are managing subscriptions between two objects. A similar result to the left join can be achieved programatically using nested queries. I’ve seen many a PHP script that uses nested loops of SQL queries to accomplish a similar result. However performing these joins programtically is of course far slower than using a database that has been specially designed for the purpose. Using a query is simpler too, once you know how it works.
As with the standard join, its important to know when a left join is applicable. There ae many situations where the use of a left join is applicable, but i’ll just explain the simple, most obvious and most common ones I come across almost daily.
Unreliable Joins
This is a term I have made up. Please feel free to suggest a more technically correct word but this does it for me. An unreliable join is where we have a relationship between two objects but the relationship is optional – it does not always exist. Its ‘unreliable’.