StackOverflow question #44686852, MySQL

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DROP TABLE tbl;
    CREATE TABLE tbl (txt TEXT);
    
    INSERT INTO tbl (txt) VALUES('<string1>//<stringA>//<string6>//2017//06//21//<stringF>//file1.txt');
    INSERT INTO tbl (txt) VALUES('<string2>//<stringB>//<string7>//2017//05//22//<stringG>//file2.txt');
    INSERT INTO tbl (txt) VALUES('<string3>//<stringC>//<string8>//2017//06//21//<stringH>//file3.txt');
    INSERT INTO tbl (txt) VALUES('<string4>//<stringD>//<string9>//2017//07//23//<stringI>//file4.txt');
    INSERT INTO tbl (txt) VALUES('<string5>//<stringE>//<string10>//2017//05//23//<stringJ>//file5.txt');

SELECT datepart, 
           COUNT(*) AS occurrences
    FROM
    (SELECT CONCAT(
         LEFT(SUBSTRING_INDEX(txt, '//', -5), INSTR(SUBSTRING_INDEX(txt, '//', -5), '//') - 1),
         '/',
         LEFT(SUBSTRING_INDEX(txt, '//', -4), INSTR(SUBSTRING_INDEX(txt, '//', -4), '//') - 1),
         '/',
         LEFT(SUBSTRING_INDEX(txt, '//', -3), INSTR(SUBSTRING_INDEX(txt, '//', -3), '//') - 1))
       AS datepart
     FROM tbl) subq
    GROUP BY datepart
    ORDER BY datepart;


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