Use the DISTINCT keyword right after SELECT to clean up redundant rows in your results.
Essential when using aggregates. It tells the database how to bucket the results (e.g., "Show me the total sales per region").
Don't try to fit a "ten-pound sack in a five-pound box." Ensure you aren't trying to store long text in a short character field. SQL Queries for Mere Mortals
Finding where two groups overlap (e.g., customers who also have active orders).
Includes all records from one table, even if there is no match in the other (useful for finding "missing" data, like products that haven't sold). 4. Summarizing and Grouping Use the DISTINCT keyword right after SELECT to
Combining two groups together (e.g., a list of all customers and all vendors).
A "solid" query follows a logical sequence. Expert reviewers from I Programmer suggest a step-by-step translation process: SQL Clause Human Translation What do you want to see? "Give me the names and prices..." FROM Where is it stored? "...from the Products table." WHERE How do you filter it? "...but only those that cost more than $50." ORDER BY How should it be sorted? "...and show the most expensive ones first." 3. Handling Relationships (JOINs) Don't try to fit a "ten-pound sack in a five-pound box
Use these to total up sales or count your users.