MySql: check and remove duplicate rows
1. How can I check if a table contains duplicate rows, with the same value of the couple (date, time) ?

SELECT *  FROM db.table HAVING COUNT(symbol) &gt; 1 


2. How can I remove all but one row ?

a) If you want to keep the row with the lowest id value:

DELETE n1 FROM table n1, table n2 WHERE n1.id &gt; n2.id AND n1.symbol= n2.symbol


b) If you want to keep the row with the highest id value:

DELETE n1 FROM table n1, table n2 WHERE n1.id &lt; n2.id AND n1.symbol= n2.symbol


3. Id is a sequential number that I increment after every post, how can I restore it after removing a row ?

Please try http://befused.com/mysql/reset-auto-increment