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    <p>Unfortunately, MySQL does not have windowing functions which is what you will need. So you will have to use something like this:</p> <p><strong>Final Query</strong></p> <pre><code>select data, group_row_number, overall_row_num from ( select data, @num := if(@data = `data`, @num + 1, 1) as group_row_number, @data := `data` as dummy, overall_row_num from ( select data, @rn:=@rn+1 overall_row_num from yourtable, (SELECT @rn:=0) r ) x order by data, overall_row_num ) x order by overall_row_num </code></pre> <p>see <a href="http://sqlfiddle.com/#!2/bb591/33" rel="noreferrer">SQL Fiddle with Demo</a></p> <p><strong>Explanation:</strong></p> <p>First, inner select, this applies a mock <code>row_number</code> to all of the records in your table (See <a href="http://sqlfiddle.com/#!2/bb591/34" rel="noreferrer">SQL Fiddle with Demo</a>):</p> <pre><code>select data, @rn:=@rn+1 overall_row_num from yourtable, (SELECT @rn:=0) r </code></pre> <p>Second part of the query, compares each row in your table to the next one to see if it has the same value, if it doesn't then start the <code>group_row_number</code> over (see <a href="http://sqlfiddle.com/#!2/bb591/35" rel="noreferrer">SQL Fiddle with Demo</a>):</p> <pre><code>select data, @num := if(@data = `data`, @num + 1, 1) as group_row_number, @data := `data` as dummy, overall_row_num from ( select data, @rn:=@rn+1 overall_row_num from yourtable, (SELECT @rn:=0) r ) x order by data, overall_row_num </code></pre> <p>The last select, returns the values you want and places them back in the order you requested:</p> <pre><code>select data, group_row_number, overall_row_num from ( select data, @num := if(@data = `data`, @num + 1, 1) as group_row_number, @data := `data` as dummy, overall_row_num from ( select data, @rn:=@rn+1 overall_row_num from yourtable, (SELECT @rn:=0) r ) x order by data, overall_row_num ) x order by overall_row_num </code></pre>
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    1. COJust to add some texture :) to this excellent answer, image that I've another column with an ID (incremental). Now I want to update my "num" column to be a concat between the letters on "data" and the counter you propose, such as "num" data would be: aa1, aa2, aa3, bb1, bb2, .. Anyway, SOOO MANY THKS! You already saved my day (or week, or month). In the meanwhile I'll try to update my table by myself...
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    2. CO@kairos happy to help, my suggestion is if you have additional questions, then post a new one for help. :) If this answer helped you, then be sure to mark is as accepted via the checkmark on the left.
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    3. CO+1 This would be easier with proper ROW_NUMBER() support. I recommend initializing `@rn` before executing the query, otherwise, especially as queries evolve over time, one can encounter [obscure, unpleasant behavior](http://bugs.mysql.com/bug.php?id=63959) with the initialization of user variables like this.
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