Distinct-Limit-Orderby-AndOr In database Concepts and Some Query Tips

LITTLE BIT DATABASE

     If you want to run multiple queries, use semicolon (;) at the end of the query.

     Try to use capital letters for "SELECT" "FROM" keywords. It is nice to use for a good programmer. 

DISTINCT: (SELECT DISTINCT city FROM customers.

     Distinct keyword returns every row only one time for each value.

LIMIT: It limits your results. 

     If you want to show first five results, all you need to do is typing "LIMIT 5" at the end.

     If you type "LIMIT 5,10"

          It retrieved ten pieces of data starting from 5 (5, 6, 7, ...15)

          First number is starting point

          Second number is amount of data

ORDER BY: used at the end of query.

     SELECT name, city FROM customer_all ORDER BY city, name

          It orders first by city alphabetically 

          and then if there are more then one customer which lives in same city, so it will order by name. 

               This is next criteria to sort the results

               you can have more option to order data numerically or dictionary sorting.

          DESC Sort the results from high to low or Z to A. (Z-A: descent dictionary sorting)

              Q.To find higher id number;

                    SELECT id FROM customers ORDER BY id DESC

AND: All of the tests must be true.
OR:  Only one of them need to be true.

1. Q: We want all customer' city, name and id number whose id is 1 or 2 and live in Istanbul?

           SELECT city, name, id FROM customers WHERE (id=1 or id=2) and city='Istanbul'

                       Please make sure that you use paranthesis, because it returns different results!

   IN: Instead of using 3 OR statements we can use IN keywords.
             SELECT * FROM customer_all WHERE id IN (3, 5, 127, 1346) ORDER BY id

For more click here. 

Ercan Duman
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