This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
Is there any difference between Group By and Distinct
What's the difference between GROUP BY and DISTINCT in a MySQL query?
Duplicate of
Is there any difference between GROUP BY and DISTINCT
It is already discussed here
If still want to listen here
Well group by and distinct has its own use.
Distinct is used to filter unique records out of the records that satisfy the query criteria.
Group by clause is used to group the data upon which the aggregate functions are fired and the output is returned based on the columns in the group by clause. It has its own limitations such as all the columns that are in the select query apart from the aggregate functions have to be the part of the Group by clause.
So even though you can have the same data returned by distinct and group by clause its better to use distinct. See the below example
select col1,col2,col3,col4,col5,col6,col7,col8,col9 from table group by col1,col2,col3,col4,col5,col6,col7,col8,col9
can be written as
select distinct col1,col2,col3,col4,col5,col6,col7,col8,col9 from table
It makes you life easier when you have more columns in the select list. But at the same time if you need to display sum(col10) along with the above columns than you will have to use Group By. In that case distinct will not work.
eg
select col1,col2,col3,col4,col5,col6,col7,col8,col9,sum(col10) from table group by col1,col2,col3,col4,col5,col6,col7,col8,col9
Hope this helps.
DISTINCT works only on the entire row. Don't be mislead into thinking SELECT DISTINCT(A), B does something different. This is equivalent to SELECT DISTINCT A, B.
On the other hand GROUP BY creates a group containing all the rows that share each distinct value in a single column (or in a number of columns, or arbitrary expressions). Using GROUP BY you can use aggregate functions such as COUNT and MAX. This is not possible with DISTINCT.
If you want to ensure that all rows in your result set are unique and you do not need to aggregate then use DISTINCT.
For anything more advanced you should use GROUP BY.
Another difference that applies only to MySQL is that GROUP BY also implies an ORDER BY unless you specify otherwise. Here's what can happen if you use DISTINCT:
SELECT DISTINCT a FROM table1
Results:
2
1
But using GROUP BY the results will come in sorted order:
SELECT a FROM table1 GROUP BY a
Results:
1
2
As a result of the lack of sorting using DISTINCT is faster in the case where you can use either. Note: if you don't need the sorting with GROUP BY you can add ORDER BY NULL to improve performance.
Care to look at the docs:
http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html
and
http://dev.mysql.com/doc/refman/5.0/en/select.html
Related
My database is called: (training_session)
I try to print out some information from my data, but I do not want to have any duplicates. I do get it somehow, may someone tell me what I do wrong?
SELECT DISTINCT athlete_id AND duration FROM training_session
SELECT DISTINCT athlete_id, duration FROM training_session
It works perfectly if i use only one column, but when I add another. it does not work.
I think you misunderstood the use of DISTINCT.
There is big difference between using DISTINCT and GROUP BY.
Both have some sort of goal, but they have different purpose.
You use DISTINCT if you want to show a series of columns and never repeat. That means you dont care about calculations or group function aggregates. DISTINCT will show different RESULTS if you keep adding more columns in your SELECT (if the table has many columns)
You use GROUP BY if you want to show "distinctively" on a certain selected columns and you use group function to calculate the data related to it. Therefore you use GROUP BY if you want to use group functions.
Please check group functions you can use in this link.
https://dev.mysql.com/doc/refman/8.0/en/group-by-functions.html
EDIT 1:
It seems like you are trying to get the "latest" of a certain athlete, I'll assume the current scenario if there is no ID.
Here is my alternate solution:
SELECT a.athlete_id ,
( SELECT b.duration
FROM training_session as b
WHERE b.athlete_id = a.athlete_id -- connect
ORDER BY [latest column to sort] DESC
LIMIT 1
) last_duration
FROM training_session as a
GROUP BY a.athlete_id
ORDER BY a.athlete_id
This syntax is called IN-SELECT subquery. With the help of LIMIT 1, it shows the topmost record. In-select subquery must have 1 record to return or else it shows error.
MySQL's DISTINCT clause is used to filter out duplicate recordsets.
If your query was SELECT DISTINCT athlete_id FROM training_session then your output would be:
athlete_id
----------
1
2
3
4
5
6
As soon as you add another column to your query (in your example, the column called duration) then each record resulting from your query are unique, hence the results you're getting. In other words the query is working correctly.
Could someone explain why the following query throws an error, if I am trying to get the names of all customers along with the total number of customers?
SELECT name, COUNT(*)
FROM CUSTOMER
I know that selecting columns along with an aggregate function requires a GROUP BY statement containing all the column names, but I don't understand the logical principle behind this.
edit:
http://sqlfiddle.com/#!2/90233/595
I guess 'error' isn't quite right, but notice how the current query returns Allison 9 as the only result.
I don't understand why it doesn't return:
Alison 9
Alison 9
Alison 9
Alison 9
Jason 9
...
(This is a new answer based on the comment and looking at the fiddle.)
The issue here is how mysql handles aggregate functions -- which is a non-standard way and different then everyone else.
mysql lets you use aggregate functions (count() is an example of an aggregate function) without a group by. All (or most?) other sql implementations require the group by when you use count(*). When you have a group by you have to say the range in the group by (for example group by name). Also every column has to be in the range or the result of an aggregate function.
SINCE you don't have a range mysql assumes the whole table and since you have a column that is not the result of a aggregate function or in the range (in this case name) mysql does something to make that column the result of an aggregate function. I'm not sure if it is specified in mysql what it does -- lets say "max()". (Fairly sure it is max()). So the real sql that is getting executed is
SELECT ANY_VALUE(name), COUNT(*)
FROM CUSTOMER
Thus you only see one name.
mysql documentation - http://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html
After reading the above I see that mysql will use the default aggregate function ANY_VALUE() for columns which are not in the range.
If you just want the total number of customers on each row you could do this
SELECT DISTINCT NAME, COUNT(NAME) OVER () AS CustomerCount
FROM CUSTOMER
In this way you don't need the GROUP BY syntax. Under the covers it is probably doing the same thing as #GordonLinoff 's answer.
I added this because maybe it makes it clearer how group by works.
Select name, Count(*) as 'CountCustomers'
FROM CUSTOMER
Group by name
Order by name
Think of it as giving an instruction of which field to aggregate by. For example, if you had a field with the State of the Customer, you could group by State which would give a count of customers by state.
Also, note you can have multiple aggregate functions in the same select using the "over (partition by" construct.
If you want the names along with the total number of customers, then use a window function:
select name, count(*) as NumCustomersWithName,
sum(count(*)) over () as NumCustomers
from customer
group by name;
Edit:
You actually seem to want:
select name, count(*) over () as NumCustomers
from customer;
In MySQL, you would do this with a subquery:
select name, cnt
from customers cross join
(select count(*) as cnt from customers) x;
The reason your query doesn't work is because it is an aggregation query that returns exactly one row. When you use aggregation functions without a GROUP BY, then the query always returns exactly one row.
I'm doing something like follows (Example, getting distinct people named "Mark" by State):
Select count(distinct FirstName) FROM table
GROUP BY State
I think the group by query organization is done first, such that the distinct is only relative to each "group by"? Basically, can "Mark" show up as a "distinct" count in each group? This would "scope" my distinct expression to the group by rows only, I believe...
This may actually depend on where DISTINCT is used. For example, SELECT DISTINCT COUNT( would be different than SELECT COUNT(DISTINCT.
In this case, it will work as you want and get a count of distinct names in each group (even if the names are not distinct across groups).
Your understanding is correct. Group by says, essentially, to take a group of rows and aggregate them into one row (based on the criteria). All aggregation functions -- including count(distinct) -- summarize values in this group.
As a note, you are using the word "scope". Just so you know, this has a particular meaning in SQL. The meaning refers to the portions of the query where a column or table alias are understood by the compiler.
Is it possible to have count in the select clause with a group by which is suppressed in the count? I need the count to ignore the group by clause
I got this query which is counting the total entries. The query is generic generated and therefore I can't make any comprehensive changes like subqueries etc.
In some specific cases a group by is needed to retrieve the correct rows and because of this the group by can't be removed
SELECT count(dv.id) num
FROM `data_voucher` dv
LEFT JOIN `data_voucher_enclosure` de ON de.data_voucher_id=dv.id
WHERE IF(de.id IS NULL,0,1)=0
GROUP BY dv.id
Is it possible to have count in the select clause with a group by which is suppressed in the count? I need the count to ignore the group by clause
well, the answer to your question is simply you can't have an aggregate that works on all the results, while having a group by statement. That's the whole purpose of the group by to create groups that change the behaviour of aggregates:
The GROUP BY clause causes aggregations to occur in groups (naturally) for the columns you name.
cf this blog post which is only the first result I found on google on this topic.
You'd need to redesign your query, the easiest way being to create a subquery, or a hell of a jointure. But without the schema and a little context on what you want this query to do, I can't give you an alternative that works.
I just can tell you that you're trying to use a hammer to tighten a screw...
Have found an alternative where COUNT DISTINCT is used
SELECT count(distinct dv.id) num
FROM `data_voucher` dv
LEFT JOIN `data_voucher_enclosure` de ON de.data_voucher_id=dv.id
WHERE IF(de.id IS NULL,0,1)=0
so suppose I do
SELECT * FROM table t GROUP BY t.id
Suppose there are multiple rows in the table with the same id, only one row of that id will ultimately come out...I suppose mysql will order the results that have the same id and the return the first one or something....my question is...how does mysql perform this ordering and is there a way that I can control its ordering so that for instance, it uses a certain field etc?
You can GROUP BY several different columns to arrange the order for which the result is grouped.
SELECT * FROM table t GROUP BY t.id, t.foo, t.bar
In strictly correct SQL, when you use GROUP BY all the values being selected must be either columns named in the GROUP BY clause or aggregate functions. MySQL allows you to violate this rule, unless you set the only_full_group_by mode. But although it allows you to perform such queries, it doesn't specify which row will be selected for each grouped column.
If you want to select a row that corresponds to the max or min of some other column, you can do something like this:
SELECT a.*
FROM table a
JOIN (SELECT id, max(somecol) maxcol
FROM table
GROUP BY id) b
ON a.id = b.id
AND a.somecol = b.maxcol
Note that this can still return multiple rows per ID if they both have the max value. You can add a final GROUP BY clause and it will select one of them arbitrarily.
When using this MySQL extension to the standard SQL GROUP BY functionality you cannot control which values for the non-aggregated, non-GROUP BY columns are selected by the server. The MySQL documentation discusses this specific case and states clearly that
The server is free to choose any value from each group, so unless they
are the same, the values chosen are indeterminate. Furthermore, the
selection of values from each group cannot be influenced by adding an
ORDER BY clause.