Perform joins carefully

Joining data from two tables in one query is a very powerful technique, particularly when combined with normalisation. However, if not done right - even by a small margin - you can incur a serious speed hit. To get good performance from your joins, follow these simple rules:

• If two fields contain identical information in different tables, declare them with the same name and with the same type
  
• Filter the query as best as you can, otherwise you can get a very large number of results. For example, joining just three tables of 50 rows each will produce 125,000 records (50x50x50), as MySQL will return every combination of the rows.
  
• Remember that it is sometimes better to have a little data duplication in exchange for the chance to not have slow joins. If your goal is maximum speed, be prepared to break a few rules!
  
• Try to use numbers as opposed to strings when comparing rows in joins - the last thing you want is thousands of strings being compared.
  
• Avoid joining rows where you are comparing non-indexed fields