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.
Related
I am trying to do a simple test where I'm pulling from a table the information of a specific part number as such:
SELECT *
FROM table_name
WHERE part_no IN ('abc123')
This returns 25 rows. Now I want to count the number that meet the "accepted" condition in a specific column but the result is limited to only the 10 most recent. My approach is to write it as follows:
Select Count(*)
FROM table_name
WHERE part_no IN ('abc123') AND lot IN ('accepted')
ORDER BY date DESC
LIMIT 10
I'm having a hard time to get the ORDER BY and LIMIT operations to work. I could use help just getting it to limit appropriately, and I can figure out the rest from there.
Edit: I understand that the operations are happening on the COUNT which only returns one row with a value; but I put the second clip to show where I am stuck in my thought process.
Your query SELECT Count(*) FROM ... will always return exactly one row.
It's not 100% clear what exactly you want to do, but if you want to know how many of the last 10 have been accepted, you could use a subquery - something like:
SELECT COUNT(*) FROM (
SELECT lot
FROM table_name
WHERE part_no IN ('abc123')
ORDER BY date DESC
LIMIT 10
)
WHERE lot IN ('accepted')
The inner query will return the 10 most recent rows for part abc123, then the outer query will count the accepted ones.
There are also other solution (for example, you could have the inner query output a field that is 0 when the part is not accepted and 1 when the part is accepted, then take the sum). Depending on which exact dialect/database you are using, you may also have more elegant options.
Select count returns ONE ROW therefore the ORDER BY and the LIMIT will not work on the results
I'm trying to get the highest version within a group. My query:
SELECT
rubric_id,
max(version) as version,
group_id
FROM
rubrics
WHERE
client_id = 1
GROUP BY
group_id
The Data:
The Results:
The rubric of ID 2 does not have a version of 2, why is this being mismatched? What do I need to do to correct this?
Edit, not a duplicate:
This is not a duplicate of SQL Select only rows with Max Value on a Column , which is a post I have read and referenced before writing this. My question is not how to find the max, my question is why is the version not matched to the correct ID
MySQL is confusing you by letting you get away with having a column in your select that isn't in your group by. To resolve the issue, make sure you don't select any field that isn't in the group by.
Instead of trying to get everything in one statement, you will need to use a subquery to find the max_version_id and then join to it.
SELECT T.*
FROM rubrics T
JOIN
(
SELECT
group_id,
max(version) as max_version
FROM
rubrics
GROUP BY
group_id
) dedupe
on T.group_id = dedupe.group_id
and T.version_id = dedupe.max_version_id
WHERE
T.client_id = 1
Edit: So MySQL allows it, but I don't think it's a good practise to use it.
You are trying to query non-aggregated data from an aggregated query. You should not do that.
A GROUP BY takes the field it should make group of rows with (in your case, what you say with your GROUP BY is: give me a result per different group_id) and gives a result (the aggregated data) based on the grouping.
Here, you try to access non aggregated data (rubric_id in your case). For some reason, the query does not crash and picks a "random" id in your aggregated data.
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.
I'm writing a query where I group a selection of rows to find the MIN value for one of the columns.
I'd also like to return the other column values associated with the MIN row returned.
e.g
ID QTY PRODUCT TYPE
--------------------
1 2 Orange Fruit
2 4 Banana Fruit
3 3 Apple Fruit
If I GROUP this table by the column 'TYPE' and select the MIN qty, it won't return the corresponding product for the MIN row which in the case above is 'Apple'.
Adding an ORDER BY clause before grouping seems to solve the problem. However, before I go ahead and include this query in my application I'd just like to know whether this method will always return the correct value. Is this the correct approach? I've seen some examples where subqueries are used, however I have also read that this inefficient.
Thanks in advance.
Adding an ORDER BY clause before grouping seems to solve the problem. However, before I go ahead and include this query in my application I'd just like to know whether this method will always return the correct value. Is this the correct approach? I've seen some examples where subqueries are used, however I have also read that this inefficient.
No, this is not the correct approach.
I believe you are talking about a query like this:
SELECT product.*, MIN(qty)
FROM product
GROUP BY
type
ORDER BY
qty
What you are doing here is using MySQL's extension that allows you to select unaggregated/ungrouped columns in a GROUP BY query.
This is mostly used in the queries containing both a JOIN and a GROUP BY on a PRIMARY KEY, like this:
SELECT order.id, order.customer, SUM(price)
FROM order
JOIN orderline
ON orderline.order_id = order.id
GROUP BY
order.id
Here, order.customer is neither grouped nor aggregated, but since you are grouping on order.id, it is guaranteed to have the same value within each group.
In your case, all values of qty have different values within the group.
It is not guaranteed from which record within the group the engine will take the value.
You should do this:
SELECT p.*
FROM (
SELECT DISTINCT type
FROM product p
) pd
JOIN p
ON p.id =
(
SELECT pi.id
FROM product pi
WHERE pi.type = pd.type
ORDER BY
type, qty, id
LIMIT 1
)
If you create an index on product (type, qty, id), this query will work fast.
It's difficult to follow you properly without an example of the query you try.
From your comments I guess you query something like,
SELECT ID, COUNT(*) AS QTY, PRODUCT_TYPE
FROM PRODUCTS
GROUP BY PRODUCT_TYPE
ORDER BY COUNT(*) DESC;
My advice, you group by concept (in this case PRODUCT_TYPE) and you order by the times it appears count(*). The query above would do what you want.
The sub-queries are mostly for sorting or dismissing rows that are not interested.
The MIN you look is not exactly a MIN, it is an occurrence and you want to see first the one who gives less occurrences (meaning appears less times, I guess).
Cheers,
I am trying to order a query by two keys. The query is built with several subqueries. The table contains, beside columns with other data, two columns, Key and Key_Father. So I need to order the results since SQL to print the results in a report. This is an example:
Key Key_Father
4 NULL
1 4
2 4
7 NULL
1 7
2 7
As you can see is a structure father-son, where a row is a father if the Key_Father is NULL and the Key column start from one for each son with a different father.
The first subquery gives the data in order, because is stored on that order in the table, but the second subquery that uses a group by, no. So I tried adding a extra column with Row_Number on the first subquery to keep that order, but the second subquery does the same thing.
This is the query:
SELECT Orden,INV_Key,Key_Padre,INV.INV_ID,INV.BOD_Bodega_ID,
CASE WHEN MAX(HIS_Ventas) > 0 OR max(HIS_Disponible) > 0 THEN 1 ELSE 0 END AS Participacion,MAX(ISNULL(HIS_Ventas,0)) AS Ventas
FROM(SELECT ROW_NUMBER() OVER (ORDER BY C.INV_Compra_ID) Orden,C.BOD_Bodega_ID,INV_Key,Key_Padre,CD.INV_ID
FROM dbo.INV_COMPRAS_USADOS C
INNER JOIN dbo.INV_COMPRAS_USADOS_DET CD ON C.INV_Compra_ID = CD.INV_Compra_ID
WHERE C.INV_Compra_ID = #Compra_ID
AND ((Key_Padre IS NULL AND CD.INV_Catalogo_Codigo = ISNULL(#Cod_Catalogo,CD.INV_Catalogo_Codigo)
AND INV_Key IN (SELECT DISTINCT Key_Padre
FROM dbo.INV_COMPRAS_USADOS_DET
WHERE INV_Compra_ID = #Compra_ID AND Key_Padre IS NOT NULL))
OR Key_Padre IN (SELECT DISTINCT INV_Key
FROM dbo.INV_COMPRAS_USADOS_DET
WHERE INV_Compra_ID = #Compra_ID AND (Key_Padre IS NULL AND CD.INV_Catalogo_Codigo = ISNULL(#Cod_Catalogo,CD.INV_Catalogo_Codigo))))) INV
LEFT JOIN DBO.HIS_HISTORICO_DETALLE HD ON INV.INV_ID = HD.INV_ID AND HD.BOD_Bodega_ID = INV.BOD_Bodega_ID
LEFT JOIN DBO.HIS_HISTORICO_INVENTARIO H on H.HIS_Historico_ID= HD.HIS_Historico_ID AND (CONVERT(datetime,(convert(varchar(20),HIS_Historico_Ano) + '/' + convert(varchar(20),HIS_Historico_Mes) + '/01')) BETWEEN #FechaDesde AND #FechaHasta)
WHERE H.HIS_Historico_Mes IS NOT NULL OR INV.INV_ID IS NULL
GROUP BY Orden,INV_Key,Key_Padre,INV.INV_ID,INV.BOD_Bodega_ID,HIS_Historico_Ano,HIS_Historico_Mes
Another interesting thing (well for me) is that when I change the #Variables for Constant values, the second query keeps the correct order, even when the constant values are the same that the #variables. This is just a portion of the total query, is a subquery that needs of another two selects, and I need to keep the order from those selects too.
So I hope that someone could help me with this. Thanks!
To order the results you need to place an ORDER BY clause on the outermost SELECT statement. Using ORDER BY in a nested SELECT is generally not permitted but even if you work around it (e.g. by using TOP), you can't rely on the results being ordered in any particular way.
Without an ORDER BY the results may appear to be coming out in the order you want but this cannot be relied upon. Running the same query on a different server or at some point in the future may produce a different order where differences in statistics, server load, etc can affect how the query optimizer actually executes the statement.
The portion of the query you've provided is outputting the following columns. Which are the ones you want to order by?
Orden (although this is just an alias for INV_Compra_ID as far as orderin is concerned)
INV_Key
Key_Padre
INV_ID
BOD_Bodega_ID
Participacion
Ventas
Let's say you want to order by just thre of them, then you need to append the following clause to the outermost SELECT:
ORDER BY
Orden,
INV_Key,
Key_Padre,
This should do it. I'm not sure if I'm missing an obvious simplification though.
ORDER BY ISNULL(Key_Father,[Key]), ISNULL(Key_Father,-1),[Key]