What difference it makes if I use, winner IN ('Subject1','Subject2'); & winner='Subject1' OR winner='Subject2';
Queries for the table 17 in the below link:
https://www.w3resource.com/sql-exercises/sql-retrieve-from-table.php#SQLEDITOR
For lists with two elements it doesn't make a difference.
However, MySQL optimizes IN when the list consists of constant expressions. It basically sorts them and does a binary search through the list. This can be a considerable savings with longer lists. As the documentation explains:
If all values are constants, they are evaluated according to the type
of expr and sorted. The search for the item then is done using a
binary search. This means IN is very quick if the IN value list
consists entirely of constants.
In general, IN is safer and does a better job of capturing the column you want. It is very easy to take conditions like this:
where winner = 'Subject1' OR winner = 'Subject2'
and add another condition:
where winner = 'Subject1' or winner = 'Subject2' and
foo = 'bar'
and this logic is probably not longer what you really want -- because it really means:
where winner = 'Subject1' or
(winner = 'Subject2' and foo = 'bar')
This doesn't happen with IN:
where winner in ('Subject1', 'Subject2') and
foo = 'bar'
If there's an index on the column in question, IN vastly out-performs OR. Experience has shown me that the db consistently doesn't use the index when there's an OR on the column.
If there's no index on the column in question, IN out-performs OR if the list is longer than about 5 (it's faster to do a few serial comparisons than traverse a small BTree of values, which is what the DB turns the list into for execution).
IN is also preferred for readability and avoiding SQL's operator precedence trap if brackets are omitted, ie x = a or x = b and c = d is parsed as x = a or (x = b and c = d) instead of the (perhaps) expected (x = a or x = b) and c = d.
Careful when using NOT:
select col1 from
(
select 1 as col1
union all
select 2 as col1
union all
select 3 as col1
union all
select 4 as col1
)x
where x.col1 NOT IN (2,3,4) ;
----------
col1
1
However
select col1 from
(
select 1 as col1
union all
select 2 as col1
union all
select 3 as col1
union all
select 4 as col1
)x
where x.col1 != 2 OR x.col1 != 3 OR x.col1 != 4 ;
---
col1
1
2
3
4
Related
I have data in MySQL table, my data looks like
Key, value
A 1
A 2
A 3
A 6
A 7
A 8
A 9
B 1
B 2
and I want to group it based on the continuous sequence. Data is sorted in the table.
Key, min, max
A 1 3
A 6 9
B 1 2
I tried googling it but could find any solution to it. Can someone please help me with this.
This is way easier with a modern DBMS that support window functions, but you can find the upper bounds by checking that there is no successor. In the same way you can find the lower bounds via absence of a predecessor. By combining the lowest upper bound for each lower bound we get the intervals.
select low.keyx, low.valx, min(high.valx)
from (
select t1.keyx, t1.valx from t t1
where not exists (
select 1 from t t2
where t1.keyx = t2.keyx
and t1.valx = t2.valx + 1
)
) as low
join (
select t3.keyx, t3.valx from t t3
where not exists (
select 1 from t t4
where t3.keyx = t4.keyx
and t3.valx = t4.valx - 1
)
) as high
on low.keyx = high.keyx
and low.valx <= high.valx
group by low.keyx, low.valx;
I changed your identifiers since value is a reserved world.
Using a window function is way more compact and efficient. If at all possible, consider upgrading to MySQL 8+, it is superior to 5.7 in so many aspects.
We can create a group by looking at the difference between valx and an enumeration of the vals, if there is a gap the difference increases. Then, we simply pick min and max for each group:
select keyx, min(valx), max(valx)
from (
select keyx, valx
, valx - row_number() over (partition by keyx order by valx) as grp
from t
) as tt
group by keyx, grp;
Fiddle
new to SQL.
I have the following set of data
A X Y Z
1 Wind 1 1
2 Wind 2 1
3 Hail 1 1
4 Flood 1 1
4 Rain 1 1
4 Fire 1 1
I would like to select all distinct 'A' fields where for all rows that contain A have flood and rain.
So in this example, the query would return only the number 4 since for the set of all rows that contain A = 4 we have Flood and Rain.
I need the values of A where for a given value 'a' in A, there exists rows with 'a' that must contain all of the following fields provided (in the example Flood and Rain).
Please let me know if you need further clarification.
I need the values of A where for a given value 'a' in A, there exists rows with 'a' that must contain all of the following fields provided (in the example Flood and Rain).
You can use aggregation, and filter with a having clause:
select a
from mytable t
where x in ('Flood', 'Rain') -- either one or the other
having count(*) = 2 -- both match
If tuples (a, x) tuples are not unique, then you want having count(distinct x) = 2 instead.
You Shooud use count(distinct X) group by A and having
count(distinct...) avoid situation where you have two time the same value for X
select A
from my_table
WHERE x in ('Flood', 'Rain')
group A
having count(distinct X) = 2
I am trying to update my table like this:
Update MyTable
SET value = 1
WHERE game_id = 1,
x =-4,
y = 8
SET value = 2
WHERE game_id = 1,
x =-3,
y = 7
SET value = 3
WHERE game_id = 2,
x = 5,
y = 2
I can do a foreach() but that will send over 50 separate Queries which is very slow.
That's why I want it to be combined into 1 big Query.
( I do use an id for each row but the combination of game_id, x and y is what I use to Identify the row I need. )
The update_batch() function from codeIgniter described here:
Update batch with CodeIgniter
was helpful and almost perfect but it only allows for 1 single where clause, you cannot (as far as I understood and tried) enter an array with multiple where clauses.
I've also checked out this question:
MYSQL UPDATE SET on the Same Column but with multiple WHERE Clauses
But it only allows for multiple row updates containing only a single different WHERE clause and I need multiple WHERE clauses! :)
Anwsers can be in simple SQL or with the use of php (and CodeIgniter) or in a different way. I'd this problem to be solved in any possible way ;)
I can really use the advice/help! =D
give this a try by using CASE
Update MyTable
SET value = CASE
WHEN game_id = 1 AND x = -4 AND y = 8 THEN 1
WHEN game_id = 1 AND x = -3 AND y = 7 THEN 2
WHEN game_id = 2 AND x = 5 AND y = 2 THEN 3
ELSE value
END
WHERE game_ID IN (1,2,3) AND -- the purpose of this WHERE clause
x IN (-4, -3, 5) AND -- is to optimize the query by preventing from
y IN (8,7,2) -- performing full table scan.
The problem:
We have a number of entries within a table but we are only interested in the ones that appear in a given sequence. For example we are looking for three specific "GFTitle" entries ('Pearson Grafton','Woolworths (P and O)','QRX - Brisbane'), however they have to appear in a particular order to be considered a valid route. (See image below)
RowNum GFTitle
------------------------------
1 Pearson Grafton
2 Woolworths (P and O)
3 QRX - Brisbane
4 Pearson Grafton
5 Woolworths (P and O)
6 Pearson Grafton
7 QRX - Brisbane
8 Pearson Grafton
9 Pearson Grafton
So rows (1,2,3) satisfy this rule but rows (4,5,6) don't even though the first two entries (4,5) do.
I am sure there is a way to do this via CTE's but some help would be great.
Cheers
This is very simple using even good old tools :-) Try this quick-and-dirty solution, assuming your table name is GFTitles and RowNumber values are sequential:
SELECT a.[RowNum]
,a.[GFTitle]
,b.[GFTitle]
,c.[GFTitle]
FROM [dbo].[GFTitles] as a
join [dbo].[GFTitles] as b on b.RowNumber = a.RowNumber + 1
join [dbo].[GFTitles] as c on c.RowNumber = a.RowNumber + 2
WHERE a.[GFTitle] = 'Pearson Grafton' and
b.[GFTitle] = 'Woolworths (P and O)' and
c.[GFTitle] = 'QRX - Brisbane'
Assuming RowNum has neither duplicates nor gaps, you could try the following method.
Assign row numbers to the sought sequence's items and join the row set to your table on GFTitle.
For every match, calculate the difference between your table's row number and that of the sequence. If there's a matching sequence in your table, the corresponding rows' RowNum differences will be identical.
Count the rows per difference and return only those where the count matches the number of sequence items.
Here's a query that implements the above logic:
WITH SoughtSequence AS (
SELECT *
FROM (
VALUES
(1, 'Pearson Grafton'),
(2, 'Woolworths (P and O)'),
(3, 'QRX - Brisbane')
) x (RowNum, GFTitle)
)
, joined AS (
SELECT
t.*,
SequenceLength = COUNT(*) OVER (PARTITION BY t.RowNum - ss.RowNum)
FROM atable t
INNER JOIN SoughtSequence ss
ON t.GFTitle = ss.GFTitle
)
SELECT
RowNum,
GFTitle
FROM joined
WHERE SequenceLength = (SELECT COUNT(*) FROM SoughtSequence)
;
You can try it at SQL Fiddle too.
*Hey everyone, I am working on a query and am unsure how to make it process as quickly as possible and with as little redundancy as possible. I am really hoping someone there can help me come up with a good way of doing this.
Thanks in advance for the help!*
Okay, so here is what I have as best I can explain it. I have simplified the tables and math to just get across what I am trying to understand.
Basically I have a smallish table that never changes and will always only have 50k records like this:
Values_Table
ID Value1 Value2
1 2 7
2 2 7.2
3 3 7.5
4 33 10
….50000 44 17.2
And a couple tables that constantly change and are rather large, eg a potential of up to 5 million records:
Flags_Table
Index Flag1 Type
1 0 0
2 0 1
3 1 0
4 1 1
….5,000,000 1 1
Users_Table
Index Name ASSOCIATED_ID
1 John 1
2 John 1
3 Paul 3
4 Paul 3
….5,000,000 Richard 2
I need to tie all 3 tables together. The most results that are likely to ever be returned from the small table is somewhere in the neighborhood of 100 results. The large tables are joined on the index and these are then joined to the Values_Table ON Values_Table.ID = Users_Table.ASSOCIATED_ID …. That part is easy enough.
Where it gets tricky for me is that I need to return, as quickly as possible, a list limited to 10 results where value1 and value2 are mathematically operated on to return a new_ value where that new_value is less than 10 and the result is sorted by that new_value and any other where statements I need can be applied to the flags. I do need to be able to move along the limit. EG LIMIT 0,10 / 11,10 / 21,10 etc...
In a subsequent (or the same if possible) query I need to get the top 10 count of all types that matched that criteria before the limit was applied.
So for example I want to join all of these and return anything where Value1 + Value2 < 10 AND I also need the count.
So what I want is:
Index Name Flag1 New_Value
1 John 0 9
2 John 0 9
5000000 Richard 1 9.2
The second response would be:
ID (not index) Count
1 2
2 1
I tried this a few ways and ultimately came up with the following somewhat ugly query:
SELECT INDEX, NAME, Flag1, (Value1 * some_variable + Value2) as New_Value
FROM Values_Table
JOIN Users_Table ON ASSOCIATED_ID = ID
JOIN Flags_Table ON Flags_Table.Index = Users_Table.Index
WHERE (Value1 * some_variable + Value1) < 10
ORDER BY New_Value
LIMIT 0,10
And then for the count:
SELECT ID, COUNT(TYPE) as Count, (Value1 * some_variable + Value2) as New_Value
FROM Values_Table
JOIN Users_Table ON ASSOCIATED_ID = ID
JOIN Flags_Table ON Flags_Table.Index = Users_Table.Index
WHERE (Value1 * some_variable + Value1) < 10
GROUP BY TYPE
ORDER BY New_Value
LIMIT 0,10
Being able to filter on the different flags and such in my WHERE clause is important; that may sound stupid to comment on but I mention that because from what I could see a quicker method would have been to use the HAVING statement but I don't believe that will work in certain instance depending on what I want to use my WHERE clause to filter against.
And when filtering using the flags table :
SELECT INDEX, NAME, Flag1, (Value1 * some_variable + Value2) as New_Value
FROM Values_Table
JOIN Users_Table ON ASSOCIATED_ID = ID
JOIN Flags_Table ON Flags_Table.Index = Users_Table.Index
WHERE (Value1 * some_variable + Value1) < 10 AND Flag1 = 0
ORDER BY New_Value
LIMIT 0,10
...filtered count:
SELECT ID, COUNT(TYPE) as Count, (Value1 * some_variable + Value2) as New_Value
FROM Values_Table
JOIN Users_Table ON ASSOCIATED_ID = ID
JOIN Flags_Table ON Flags_Table.Index = Users_Table.Index
WHERE (Value1 * some_variable + Value1) < 10 AND Flag1 = 0
GROUP BY TYPE
ORDER BY New_Value
LIMIT 0,10
That works fine but has to run the math multiple times for each row, and I get the nagging feeling that it is also running the math multiple times on the same row in the Values_table table. My thought was that I should just get only the valid responses from the Values_table first and then join those to the other tables to cut down on the processing; with how SQL optimizes things though I wasn't sure if it might not already be doing that. I know I could use a HAVING clause to only run the math once if I did it that way but I am uncertain how I would then best join things.
My questions are:
Can I avoid running that math twice and still make the query work
(or I suppose if there is a good way
to make the first one work as well
that would be great)
What is the fastest way to do this
as this is something that will
be running very often.
It seems like this should be painfully simple but I am just missing something stupid.
I contemplated pulling into a temp table then joining that table to itself but that seems like I would trade math for iterations against the table and still end up slow.
Thank you all for your help in this and please let me know if I need to clarify anything here!
** To clarify on a question, I can't use a 3rd column with the values pre-calculated because in reality the math is much more complex then addition, I just simplified it for illustration's sake.
Do you have a benchmark query to compare against? Usually it doesn't work to try to outsmart the optimizer. If you have acceptable performance from a starting query, then you can see where extra work is being expended (indicated by disk reads, cache consumption, etc.) and focus on that.
Avoid the temptation to break it into pieces and solve those. That's an antipattern. That includes temp tables especially.
Redundant math is usually ok - what hurts is disk activity. I've never seen a query that needed CPU work reduction on pure calculations.
Gather your results and put them in a temp table
SELECT * into TempTable FROM (SELECT INDEX, NAME, Type, ID, Flag1, (Value1 + Value2) as New_Value
FROM Values_Table
JOIN Users_Table ON ASSOCIATED_ID = ID
JOIN Flags_Table ON Flags_Table.Index = Users_Table.Index
WHERE New_Value < 10)
ORDER BY New_Value
LIMIT 0,10
Return Result for First Query
SELECT INDEX, NAME, Flag1, New_Value
FROM TempTable
Return Results for count of Types
Select ID, Count(Type)
FROM TempTable
GROUP BY TYPE
Is there any chance that you can add a third column to the values_table with the pre-calculated value? Even if the result of your calculation is dependent on other variables, you could run the calculation for the whole table but only when those variables change.