mysql查询总和的差异

Question

I am having different results on what appears to be the same query. I am very confused on why the total differs from the queries. Hopefully someone has an explanation on this issue and maybe say which should be the correct total. Thank you

select sum(tm_hours) 
FROM bhds_timecard 
WHERE (tm_notes = 'Session' 
 OR tm_notes = 'Intake' 
 OR tm_notes = 'OT' 
 or tm_notes = 'PT' 
 or tm_notes = 'Speech') 
AND (tm_date BETWEEN '2016-04-01' AND '2016-04-30')

This totals 1792.25

SELECT SUM(tm_hours) 
FROM bhds_timecard 
WHERE (tm_date BETWEEN '2016-04-01' AND '2016-04-30') 
AND tm_notes = 'Session' 
OR tm_notes = 'Intake' 
OR tm_notes = 'OT' 
OR tm_notes = 'PT' 
OR tm_notes = 'Speech'

This totals 1796.25

Answer 1

If you understand why 5 + 3 * 2 = 11 then this will help understand the source of your problem. Some people may mistakenly think the above calculation should give you 16 because they are unaware of the order of operations. Multiplication takes precedence over addition so it is done first.

Likewise, AND has a higher precedence than OR so it is done first. Check out the Operator Precedence section in the MySQL reference manual.

This is likely the code you were trying to build:

SELECT SUM(tm_hours) 
FROM bhds_timecard 
WHERE tm_date BETWEEN '2016-04-01' AND '2016-04-30'
AND (tm_notes = 'Session' 
OR tm_notes = 'Intake' 
OR tm_notes = 'OT' 
OR tm_notes = 'PT' 
OR tm_notes = 'Speech');

In this case, since parentheses override the standard order of operations, everything in the parentheses gets executed first (which are all the OR statements). Therefore the entire parenthetical section will evaluate to TRUE if only one of the conditions evaluates to TRUE). Next all of your ANDS get evaluated and you have your answer.

Answer 2

Second query must be like that:

SELECT SUM(tm_hours) FROM bhds_timecard WHERE (tm_date BETWEEN '2016-04-01' AND '2016-04-30') 
AND (tm_notes = 'Session' OR tm_notes = 'Intake' OR tm_notes = 'OT' OR tm_notes = 'PT' OR tm_notes = 'Speech')

You lose the ( bracket for second group of conditions

Answer 3

The first query is evaluating all of your OR s as one condition -

WHERE (tm_notes = 'Session' 
 OR tm_notes = 'Intake' 
 OR tm_notes = 'OT' 
 or tm_notes = 'PT' 
 or tm_notes = 'Speech')
AND (tm_date BETWEEN '2016-04-01' AND '2016-04-30')

If a row meets any of those conditions, it evaluates as true and then moves on to the date constraint. So there's really only 2 top-level condition in the first query.

The second query actually has 6 conditions -

WHERE (tm_date BETWEEN '2016-04-01' AND '2016-04-30') 
AND tm_notes = 'Session' 
OR tm_notes = 'Intake' 
OR tm_notes = 'OT' 
OR tm_notes = 'PT' 
OR tm_notes = 'Speech'

If any of the 6 OR s are true, the whole WHERE clause is evaluated as true.

Answer 4

Your first query conditions are like this

Where
--first condition 
(tm_notes = 'Session' OR tm_notes = 'Intake' OR tm_notes = 'OT' or tm_notes = 'PT' or tm_notes = 'Speech')
AND 
-- second condition
(tm_date BETWEEN '2016-04-01' AND '2016-04-30')

you choose both the conditions to be true, first condition itself a combination of conditions,

While in the second query you have this many conditions with OR and so if any of the conditions gets true the whole condition pattern gets true and so there is the difference in the result

Where
-- first condition
(tm_date BETWEEN '2016-04-01' AND '2016-04-30') 
AND 
-- second condition
tm_notes = 'Session' 
OR
-- third condition 
tm_notes = 'Intake' 
OR 
-- fourth condition
tm_notes = 'OT' 
OR 
-- fifth condition
tm_notes = 'PT' 
-- sixth condition
OR 
tm_notes = 'Speech'

Answer 5

AND & OR work like + & -

a+(b*c) is not the same as (a+b)*c

Same for AND & OR

a OR (b AND c) is not the same as (a OR b) AND c

So be carefull with brackets when you mix up AND & OR