Database considerations

The end of the term is over , Record the latest database operation

A little basic rule , Think of a little, record a little
Keyword execution order
FROM WHERE GROUP HAVING SELECT ORDER LIMIT
When writing, first put the keywords you want to use , Then fill in the constraints according to the above rules to make your thinking clearer
WHERE Yes FROM Filter but does not support aggregate function filtering , because WHERE Precede GROUP BY perform
Try not to use HAVING sentence , Avoid secondary polymerization , Because it hurts memory , Try to use subquery
FROM The pseudo table generated by the subquery used later should be aliased （ My own understanding here is to generate a temporary table , Name this watch ）
UNION and UNION ALL The difference between ,UNION It's going to be weightless ,UNION ALL It's not going to be heavy , So the efficiency is higher than UNION
For the time being, I feel these are more important , I'll make up for the rest later