Where, having, group by, order by, is null, not in, subquery, delete, date function

where
Filter conditions , use and,or,not Equal operator separation

If necessary at the same time and and or What shall I do? ？ Parentheses are grouped
from Table name
where(…)
or(…)

having and group by

group by grouping ,having Filter conditions
group by …
having …

order by

Default ascending order

order Name desc For the descending order

order by a,b,c;
a,b,c The order of determines the order of sorting

Sort by location instead of by name
order by 3 descs（ In descending order with the third column ）

Judge null
I used left join query , Learned that only IS NULL or IS NOT NULL, Can't use =, <, <>, != These operators are used to judge NULL.

not in
Use not in Find out the data that is not in these results .

select name from customers where id not in (
	select customerId from orders
);

Subquery

1.select Subquery

2.where Subquery , If it is a set, it cannot be used =, And we should use in

3.from Subquery , For more than three tables
a,b,c Three tables , First check ab, The query again c

delete

Single table delete

Multi table delete ：

delete p1 from Person p1 ,Person p2
where p1.Email =p2.Email and p1.Id > p2.Id  Multi table delete , hold p1 Greater than p2 Of ID Deleted 

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datediff

DATEDIFF Is the difference set of days between two dates

DATEDIFF(expr1,expr2)

DATEDIFF（） return expr1 − expr2, Expressed as a value in days from one date to another .expr1 and expr2 It's a date or a date and time expression . Only the date part of the value is used in the calculation .

mysql> SELECT DATEDIFF(‘2007-12-31 23:59:59’,‘2007-12-30’); -> 1
mysql> SELECT DATEDIFF(‘2010-11-30 23:59:59’,‘2010-12-31’); -> -31