summary:
1. Find mysql setting parameters:
show variables like 'sql_safe_updates';
2, having for the result set is, but where is to look at the disk, there should be only having to find where existing operations in memory;
3, count (0), count (1), count (*) results are the same, count (column) exclude null, sometimes column data is converted to 0 or 1, to count by sum, rather than count, averaging reference achievement
4, sub-query sequence: where group by having order by limit
5、