mySQL: get hash value for each row?

Question

Currently I\'m manually creating a string where I concatenate all the values in each row in my table. I\'m hashing this string for each row to get a hash value for the curre

Answer 1

There are problems with CONCAT, e.g. CONCAT('ab', 'c') vs CONCAT('a', 'bc'). Two different rows, but result is the same. You could use CONCAT_WS(';', 'ab', 'c') to get ab;c but in case of CONCAT_WS(';', ';', '') vs CONCAT_WS(';', '', ';') you still get the same result.

Also CONCAT(NULL, 'c') returns NULL.

I think the best way is to use QUOTE:

SELECT MD5(CONCAT(QUOTE(c1), QUOTE(c2), QUOTE(c3))) AS row_hash FROM t1;

Result of: select (concat(quote('a'), quote('bc'), quote('NULL'), quote(NULL), quote('\''), quote('')));

is: 'a''bc''NULL'NULL'\''''

Also, don't use GROUP_CONCAT() to get hash of table, it has limit: https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_group_concat_max_len

Instead, CHECKSUM TABLE might be better, but you can't skip columns with CHECKSUM TABLE https://dev.mysql.com/doc/refman/5.7/en/checksum-table.html