Conditions inquiry:
Logical condition: and ( to ) or ( or )
and: a blog on the existing practical examples
Theorem: both sides must meet to appear (and are and mean)
or: or the meaning of
effect:
Definition: as long as any of them will be (or generally appear when using or)
Exercise:
effect:
Little Red Riding Hood can see the female gender but to meet so there is java1018 class.
<> (Not equal)
effect:
between and ( equivalent to > = and <=) :
effect:
Defined: between and is nailed only and less than or equal to
Determine sky conditions:
null: nothing is empty. Really no value
Empty string: There is value, but it is empty. It is an empty string
effect:
effect:
Definition: generally empty or, and usually is not empty (empty and not empty, but the two conditions are determined to be empty or, is not empty and).
Fuzzy query: like
%: It represents any number of characters
_: Represents a character
and like to use _ or% with the job.
effect:
Analysis: Add 'is because sname type varchar,% is added back after any large characters plus 0 ~ n can be matched, that is to say can be large at the beginning of the match.
%% Can be large write this on behalf of the word if there is a large field can be reflected
effect:
Analysis: plus _ because _ is representative of a character
E.g:
effect
Aggregate query;
Conventional polymerization functions: SUM () AVG () max () min () COUNT ()
Function only between the polymerization and select from.
sum (): sum (sum field bristling)
effect:
avg (): average
max (): maximum value
min (): minimum value
解析：ifnull（heml，0）:是让heml里的null值为0.
count（）：是统计每一列的记录数，是统计每一列不为空的记录数
最好count（*）
count（*）：会吧所有列都count一下然后取最大值
