in the below image I'm using
SELECT DISTINCT(name),date,reporting,leaving from attendance where date='2016-09-01
and I'm still getting repeating names. Why?
When using DISCTINCT, MySQL uses all columns as grouping factor. If you want group by only one column and get all corresponding column values, use GROUP BY instead
SELECT name, date, reporting, leaving FROM attendance GROUP BY name WHERE ...
Actually your all rows have distinct data apart from Name column if you want only distinct names then you can get it with help of Aggregate functions, you can use MIN or MAX as per your business requirement
SELECT Name,MAX(date),MAX(reporting),MAX(leaving)
FROM attendance
WHERE date='2016-09-01'
GROUP BY Name
Related
In a MySQL database To get the SUM of a column I just do:
SELECT SUM(orderamount_total) FROM io__order_infos_hext
Is it possible in only one SQL request to get the SUM of orderamount_total of the different curriences defined in the column currency?
You just need a group by
SELECT currency, SUM(orderamount_total)
FROM io__order_infos_hext
group by currency
Yes It's possible. Please use the GROUP BY select clause.
SELECT SUM(orderamount_total) FROM io__order_infos_hext GROUP BY your_currency_column;
I have a table :
CREATE TABLE data
(
value integer,
name varchar(100)
)
In my table there can be duplicate values of name possible with different value of value. Now I want to get DISTINCT name and there avg() value from the Table data.
I am able to get DISTINCT value of name but unable to get avg() of there values.
Now with following Query I get avg() of all data :
select floor(avg(value)) from data
I know this is incorrect but I am new to SQL. I want this select floor(avg(value)) for distinct values of name.
Data :
insert into data values(10, 'mnciitbhu')
insert into data values(20, 'mnciitbhu')
insert into data values(40, 'mafiya69')
insert into data values(20, 'mafiya69')
insert into data values(0, 'mafiya69')
Output :
mnciitbhu 15
mafiya69 20
Adding this because the other answers while accurate, are not detailed.
What you want to do here, are use the grouping and aggregation features of SQL.
grouping your results by particular fields, will divide your result set into discrete sections, which you can operate on with aggregate functions, to get averages, sums, counts etc, per group.
For a full list of aggregate functions, and other miscellaneous information about group by, you can read 12.16.1 GROUP BY (Aggregate) Functions.
In your instance, since you want the average per name, you will need to group by name. This would give the following query:
select name, avg(value)
from `data`
group by name; -- this is the important line
And this query will calculate the average of value, for each group of names in your table, returning one row per group.
One very important consideration when using group by, is that all fields contained in the select, must either be contained in the group by clause, or used in aggregate functions. If you refer to a field that isn't covered by this, you may end up with undesired indeterminate results.
From the manual 12.16.3 MySQL Handling of GROUP BY
MySQL extends the use of GROUP BY so that the select list can refer to
nonaggregated columns not named in the GROUP BY clause. This means
that the preceding query is legal in MySQL. You can use this feature
to get better performance by avoiding unnecessary column sorting and
grouping. However, this is useful primarily when all values in each
nonaggregated column not named in the GROUP BY are the same for each
group. The server is free to choose any value from each group, so
unless they are the same, the values chosen are indeterminate.
The importance of that paragraph cannot be overstated. It is very easy to mis-understand how this works, arrive at a query that seems to give the desired result, but will occasionally give incorrect/undesired data.
Use this code:
select name,AVG(value) as Average from data
group by name
order by name desc
OUTPUT:
name Average
mnciitbhu 15
mafiya69 20
Try this
select name,avg(value) from data group by name
I have table orders with fields id, customer_id and amt:
SQL Fiddle
And I want get customer_id with the largest amt and value of this amt.
I made the query:
SELECT customer_id, MAX(amt) FROM orders;
But the result of this query contained an incorrect value of customer_id.
Then I built such the query:
SELECT customer_id, MAX(amt) AS maximum FROM orders GROUP BY customer_id ORDER BY maximum DESC LIMIT 1;
and got the correct result.
But I do not understand why my first query not worked properly. What am I doing wrong?
And is it possible to change my second query to obtain the necessary information to me in a simpler and competent way?
MySQL will allow you to leave GROUP BY off of a query, thus returning the MAX(amt) in the entire table with an arbitrary customer_id. Most other RDBMS require the GROUP BY clause when using an aggregate.
I don't see anything wrong with your 2nd query -- there are other ways to do it, but yours will work fine.
Some versions of SQL give you a warning or error when you select a field, have an aggregate operator like MAX or SUM, and the field you are selecting does not appear in GROUP BY.
You need a more complicated query to fetch the customer_id corresponding to the max amt. Unfortunately SQL is not as naive as you think. Once such way to do this is:
select customer_id from orders where amt = ( select max(amt) from orders);
Although a solution using joins is likely more performant.
To understand why what you were trying to do doesn't make sense, replace MAX with SUM. From the stance of how aggregate operators are interpreted, it's a mere coincidence that MAX returns something that corresponds to an actual row. SUM does not have this property, for instance.
Practically your first query can be seen as if it were GROUP BY-ed into a big single group.
Also, MySQL is free to choose each output value from different source rows from the same group.
http://dev.mysql.com/doc/refman/5.7/en/group-by-extensions.html
MySQL extends the use of GROUP BY so that the select list can refer to
nonaggregated columns not named in the GROUP BY clause.
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. Sorting of the result set
occurs after values have been chosen, and ORDER BY does not affect
which values within each group the server chooses.
The problem with MAX() is that it will select the highest value of that specified field, considering the specified field alone. The other values in the same row are not considered or given preference for the result at any degree. MySQL will usually return whatever value is the first row of the GROUP (in this case the GROUP is composed by the entire table sinse no group was specified), dropping the information of the other rows during the agregation.
To solve this, you could do that:
SELECT customer_id, amt FROM orders ORDER BY amt DESC LIMIT 1
It should return you the customer_id and the highest amt while preserving the relation between both, because no agregation was made.
I have 2 columns in my product table -name and brand, Given is the data,
NAME BRAND
'Ruby Axe Guitar', 'Guitar''s & Co'
'TV' , 'LG'
When I tried this query its working fine,
select name,brand, sum(1000) as sum,'Test' as name1
from products
group by name,brand
but I got surprised even when I dont include brand in the group by clause the query is working fine..
select name,brand, sum(1000) as sum,'Test' as name1
from products
group by name
Can someone explain?
You cannot select ungrouped row without aggregate function - MySQL will give you random value. I guess you are lucky with this second query
Because NAME is already unique with your data, so GROUP BY NAME is same as GROUP BY NAME, OTHER_FIELD.
NAME is unique, then the combination with any other column is unique too.
MySQL is a lot less strict than it should be IMHO. According to the actual SQL specification, any non-grouped column needs an aggregate function in a query containing a GROUP BY clause.
MySQL will allow retrieving non-grouped columns without such aggregate functions, returning an arbitrary value. They have an explanation of this choice in their documentation:
MySQL extends the use of GROUP BY so that the select list can refer to
nonaggregated columns not named in the GROUP BY clause. This means
that the preceding query is legal in MySQL. You can use this feature
to get better performance by avoiding unnecessary column sorting and
grouping. However, this is useful primarily when all values in each
nonaggregated column not named in the GROUP BY are the same for each
group. The server is free to choose any value from each group, so
unless they are the same, the values chosen are indeterminate.
I believe its intended as an ignored oversight. TYPICALLY, you would be required to include any column that is a non-aggregate into the "GROUP BY" clause. However, MySQL basically grabs the first entry for the column not part of the aggregate it encounters.
This could be ok, such as doing a query against a table/columns that you know wont change no matter how many records in the corresponding group by. For example. You want a list of customers and their total orders. The orders table has a customer ID that joins to the customer table. So, you can do a SUM( Orders.Amount ), yet still get customer ID, Name, Address, Phone. Since the join is on a customer ID, the corresponding name, address, etc will never change and thus not be important within the group by. Just group by a customer ID.
So, MySQL won't choke on you if you inadvertently leave out a column...
I want to get the distinct value of a particular column however duplicity is not properly managed if more than 3 columns are selected.
The query is:
SELECT DISTINCT
ShoppingSessionId, userid
FROM
dbo.tbl_ShoppingCart
GROUP BY
ShoppingSessionId, userid
HAVING
userid = 7
This query produces correct result, but if we add another column then result is wrong.
Please help me as I want to use the ShoppingSessionId as a distinct, except when I want to use all the columns from the table, including with the where clause .
How can I do that?
The DISTINCT keyword applies to the entire row, never to a column.
Presently DISTINCT is not needed at all, because your script already makes sure that ShoppingSession is distinct: by specifying the column in GROUP BY and filtering on the other grouping column (userid).
When you add a third column to GROUP BY and it results in duplicated ShoppingSession, it means that some ShoppingSession values are associated with many different values of the added column.
If you want ShoppingSession to remain distinct after including that third column, you should decide which values of the the added column should be left in the output and which should be discarded. This is called aggregating. You could apply the MAX() function to that column, or MIN() or any other suitable aggregate function. Note that the column should not be included in GROUP BY in this case.
Here's an illustration of what I'm talking about:
SELECT
ShoppingSessionId,
userid,
MAX(YourThirdColumn) AS YourThirdColumn
FROM dbo.tbl_ShoppingCart
GROUP BY
ShoppingSessionId,
userid
HAVING userid = 7
There's one more note on your query. The HAVING clause is typically used for filtering on aggregated columns. If your filter does not involve aggregated columns, you'll be better off using the WHERE clause instead:
SELECT
ShoppingSessionId,
userid,
MAX(YourThirdColumn) AS YourThirdColumn
FROM dbo.tbl_ShoppingCart
WHERE userid = 7
GROUP BY
ShoppingSessionId,
userid
Although both queries would produce identical results, their efficiency would be different, because the first query would have to pull all rows, group/aggregate them, then discard all rows except userid = 7, but the second one would discard rows first and only then group/aggregate the remaining, which is much more efficient.
You could go even further and exclude the userid column from GROUP BY and pull its value with an aggregate function:
SELECT
ShoppingSessionId,
MAX(userid) AS userid,
MAX(YourThirdColumn) AS YourThirdColumn
FROM dbo.tbl_ShoppingCart
WHERE userid = 7
GROUP BY
ShoppingSessionId
Since all userid values in your output are supposed to contain 7 (because that's in your filter), you can just pick a maximum value per every ShoppingSession, knowing that it'll always be 7.