What is wrong with this query :
SELECT count() as total_students
FROM student_information;
SELECT
(
SELECT student_project_marks
from students_project
WHERE student_project_id=student_information.student_project_id
)
+ student_assignment_marks
+ student_exam_marks AS total_marks
FROM student_information
ORDER BY total_marks DESC;
UPDATE student_information
SET (
SELECT student_grade
FROM student_information
LIMIT 0.1*total_students
ORDER BY total_marks DESC
)="A";
Iam trying to select 0.1*total_students number of students ordered by their total marks obtained and update their grades.... Top 10% will be awarded as A.
I am getting error :
syntax error near '('
I have 2 tables :
created them via following query:
create table if not exists student_information (
student_name varchar(80),
student_roll_num int primary key,
student_email varchar(64),
student_assignment_marks int(2) check(student_assignment_marks<=30),
student_exam_marks int(2) check(student_exam_marks<=50),
student_project_id varchar(25),
student_grade varchar(2)
)
create table if not exist students_project (
student_project_id varchar(25),
student_project_title varchar(25),
student_project_marks int(2) check(student_project_marks>=0 and student_project_marks<=20)
)
Marks in the project is accessed from the student_project table via the student_project_id.
Now how do I award the grade based on the total marks...
Top 10% have to be awarded A, next 10% B and so on...
I know how to calculate total marks...
I have writen a query like this:
select student_roll_num,
(SELECT student_project_marks
from students_project
WHERE student_project_id=student_information.student_project_id )+
student_assignment_marks+student_exam_marks as total_marks from student_information;
It works.
This is not even approximately correct SQL. Also, it's not clear how you're executing these statements (from a console, from a program, etc).
A few comments to help get you on your way:
SQL does not "remember" the results of previous statements from one statement to the next. Therefore, your calculation of total_students in the first SQL statement has no connection with your attempt to actually use that value in your third statement. Similarly, your attempt to derive total_marks in the second query is not available in your third query.
Your second statement only makes sense if the internal query is guaranteed to produce only a single record for each row in student_information. I'm reasonable certain that what you're attempting to do would be better done using a JOIN rather than a sub-query.
The third query (UPDATE) is the one that's furthest away from SQL. UPDATE operates on one or more columns in a table. Each column is assigned a new value. The columns it operates on must be literally named with the correct identifiers. You can use a sub-query on the right side of the equals sign, but not on the left (although you don't have any reason to use one here). The conditions for the set of rows on which to operate belong in a WHERE clause at the end of the UPDATE statement, not within any kind of sub-query.
Guessing at your intent, I think you probably need to SUM or AVG the grades from student_projects (assuming each student has several projects to consider), and there's no aggregation in any of your queries.
Related
I have a subject table that has subject_id column. In the table I have one row that has subject_id null other than that subject_id has a distinct value.
I am looking for single query I can fetch the data on basis of subject_id.
Select * from subject where subject_id = x;
If there is no data found w.r.t x than it should return the row with subject_id = null
In general this is a terrible pattern for tables. NULL as a primary key value is only going to cause you pain and suffering in the long run. Using a NULL-keyed row as a default for when your query matches no other rows will lead to strange behavior somewhere unexpected.
The simplest way would be to simply include the NULL row as the last row of any query and then only fetch the first row. But that only works when your query can only return at most one valid result.
select *
from subject
where subject_id = ? or subject_id is null
order by subject_id asc nulls last
Possibly the biggest problem with a NULL PK for your default/placeholder row in subject is that anywhere else you have a NULL subject_id cannot simply join to that row using x.subject_id = y.subject_id.
If you really need such a row, I suggest using -1 instead of NULL as the "not exists" value. It will make your life much easier across the board, especially if you need to join to it.
Are there any DB engines that allow you to run an EXPLAIN (or other function) where it will give you an approximate count of values that may be returned before an aggregation is run (not rows scanned but that actually would be returned)? For example, in the following query:
SELECT gender, COUNT(1) FROM sales JOIN (
SELECT id, person FROM sales2 WHERE country='US'
GROUP BY person_id
) USING (id)
WHERE sales.age > 20
GROUP BY gender
Let's say this query returns 3 rows after being aggregated, but would return 170M rows if unaggregated.
Are there any tools where you can run the query to get this '170M' number or does this have to do with complexity theory (or something similar) where it's almost just as expensive to run the query (without the final aggregation/having/sort/limit/etc) to get the count? In other words, doing a rewrite to:
SELECT COUNT(1) FROM sales JOIN (
SELECT id, person FROM sales2 WHERE country='US'
GROUP BY person_id
) USING (id)
WHERE sales.age > 20
But having to execute the query nonetheless.
As an example of using the current (mysql) explain to show how 'off' it is to get what I'm looking for:
explain select * from movies where title>'a';
# rows=147900
select count(1) from _tracktitle where title>'a';
# 144647 --> OK, pretty close
explain select * from movies where title>'u';
# rows=147900
select * from movies where title>'u';
# 11816 --> Not close at all
Assuming you can use MS SQL Server, you could tap into the same data the Optimiser is using for cardinality estimation: DBCC SHOW_STATISTICS (table, index) WITH HISTOGRAM
Part of data sets you get back is per-column histogram, which is essentially number of rows for each value range found in the table.
You probably want to query the data programmatically, one way to achieve this would be to insert it into a temp table:
CREATE TABLE #histogram (
RANGE_HI_KEY datetime PRIMARY KEY,
RANGE_ROWS INT,
EQ_ROWS INT,
DISTINCT_RANGE_ROWS INT,
AVG_RANGE_ROWS FLOAT
)
INSERT INTO #histogram
EXEC ('DBCC SHOW_STATISTICS (Users, CreationDate) WITH HISTOGRAM')
SELECT 'Estimate', SUM(RANGE_ROWS+EQ_ROWS) FROM #histogram WHERE RANGE_HI_KEY BETWEEN '2010-08-30 08:28:45.070' AND '2010-09-20 22:15:33.603'
UNION ALL
select 'Actual', COUNT(1) from Users u WHERE u.CreationDate BETWEEN '2010-08-30 08:28:45.070' AND '2010-09-20 22:15:33.603'
For example, check out what this same query run against Stack Overflow Database.
| -------- | ----- |
| Estimate | 98092 |
| Actual | 11715 |
it seems like a lot but then keep in mind that the whole table has almost 15mil records.
A note on precision and other gotchas
The maximum number of histogram steps is capped at 200 - which is not a lot, so you are not getting guaranteed 10% margin of error, but neither does SQL Server.
As you insert data into table, histograms may get stale so your results would get skewed even more.
There are different ways to update this data, some are reasonably quick while others effectively require full table scan
not all columns will have statistics. You can either create it manually or (I believe) it gets created automatically if you run a search with the column as predicate
MS Sql Server offers "execution plans". In the picture below I have queries and I press (Ctrl-L) to see the plans.
In my queries I return all records in first and just the count in the other, using the same table.
Look at metric corresponding to red arrows- estimated # of rows that WILL be scanned when queries are run. In this case, that number is same regardless whether count(*) or *, your point in case!
I m using report tab -> group sort expert-> top n to get top n record but i m getting sum of value in report footer for all records
I want only sum of value of top n records...
In below image i have select top 3 records but it gives sum of all records.
The group sort expert (and the record sort expert too) intervenes in your final result after the total summary is calculated. It is unable to filter and remove rows, in the same way an ORDER BY clause of SQL cannot effect the SELECT's count result (this is a job for WHERE clause). As a result, your summary will always be computed for all rows of your detail section and, of course, for all your group sums.
If you have in mind a specific way to exlude specific rows in order to appear the appropriate sum the you can use the Select Expert of Crystal Reports to remove rows.
Alternatively (and I believe this is the best way), I would make all the necessary calculations in the SQL command and I would sent to the report only the Top 3 group sums (then you can get what you want with a simple total summary of these 3 records)
Something like that
CREATE TABLE #TEMP
(
DEP_NAME varchar(50),
MINVAL int,
RMAVAL int,
NETVAL int
)
INSERT INTO #TEMP
SELECT TOP 3
T.DEP_NAME ,T.MINVAL,T.RMAVAL,T.NETVAL
FROM
(SELECT DEP_NAME AS DEP_NAME,SUM(MINVAL) AS MINVAL,SUM(RMAVAL) AS
RMAVAL,SUM(NETVAL) AS NETVAL
FROM YOURTABLE
GROUP BY DEP_NAME) AS T
ORDER BY MINVAL DESC
SELECT * FROM #TEMP
I'm building a BI report for a client where there is a 1-n related join involved.
The joined table has a field for employee ID (EmplId).
The query that I've built for this report is supposed to give a 1 in its field "OneEmployee" if all the related posts have the same employee in the EmplId field, null if it's different employees, i.e:
TaskTrans
TaskTransHours > EmplId: 'John'
TaskTransHours > EmplId: 'John'
This should give a 1 in the said field in the query
TaskTrans
TaskTransHours > EmplId: 'John'
TaskTransHours > EmplId: 'George'
This should leave the said field blank
The idea is to create a field where a case function checks this and returns the correct value. But my problem is whereas there is a way to check for this through SQL.
select not count(*) from your_table
where employee_id = GIVEN_ID
and your_field not in ( select min(your_field)
from your_table
where employee_id = GIVEN_ID);
Note: my first idea was to use LIMIT 1 in the inner query, but MYSQL didn't like it, so min it was - the points to use any, but only one. Min should work, but the field should be indexed, then this query will actually execute rather fast, as only indexes would be used (obviously employee_id should also be indexed).
Note2: Do not get too confused with not in front of count(*), you want 1 when there is none that is different, I count different ones, and then give you the not count(*), which will be one if count is 0, otherwise 0.
Seems a job for a window COUNT():
SELECT
…,
CASE COUNT(DISTINCT TaskTransHours.EmplId) OVER () WHEN 1 THEN 1 END
AS OneEmployee
FROM …
i have loanTable that contain two field loan_id and status
loan_id status
==============
1 0
2 9
1 6
5 3
4 5
1 4 <-- How do I select this??
4 6
In this Situation i need to show the last Status of loan_id 1 i.e is status 4. Can please help me in this query.
Since the 'last' row for ID 1 is neither the minimum nor the maximum, you are living in a state of mild confusion. Rows in a table have no order. So, you should be providing another column, possibly the date/time when each row is inserted, to provide the sequencing of the data. Another option could be a separate, automatically incremented column which records the sequence in which the rows are inserted. Then the query can be written.
If the extra column is called status_id, then you could write:
SELECT L1.*
FROM LoanTable AS L1
WHERE L1.Status_ID = (SELECT MAX(Status_ID)
FROM LoanTable AS L2
WHERE L2.Loan_ID = 1);
(The table aliases L1 and L2 could be omitted without confusing the DBMS or experienced SQL programmers.)
As it stands, there is no reliable way of knowing which is the last row, so your query is unanswerable.
Does your table happen to have a primary id or a timestamp? If not then what you want is not really possible.
If yes then:
SELECT TOP 1 status
FROM loanTable
WHERE loan_id = 1
ORDER BY primaryId DESC
-- or
-- ORDER BY yourTimestamp DESC
I assume that with "last status" you mean the record that was inserted most recently? AFAIK there is no way to make such a query unless you add timestamp into your table where you store the date and time when the record was added. RDBMS don't keep any internal order of the records.
But if last = last inserted, that's not possible for current schema, until a PK addition:
select top 1 status, loan_id
from loanTable
where loan_id = 1
order by id desc -- PK
Use a data reader. When it exits the while loop it will be on the last row. As the other posters stated unless you put a sort on the query, the row order could change. Even if there is a clustered index on the table it might not return the rows in that order (without a sort on the clustered index).
SqlDataReader rdr = SQLcmd.ExecuteReader();
while (rdr.Read())
{
}
string lastVal = rdr[0].ToString()
rdr.Close();
You could also use a ROW_NUMBER() but that requires a sort and you cannot use ROW_NUMBER() directly in the Where. But you can fool it by creating a derived table. The rdr solution above is faster.
In oracle database this is very simple.
select * from (select * from loanTable order by rownum desc) where rownum=1
Hi if this has not been solved yet.
To get the last record for any field from a table the easiest way would be to add an ID to each record say pID. Also say that in your table you would like to hhet the last record for each 'Name', run the simple query
SELECT Name, MAX(pID) as LastID
INTO [TableName]
FROM [YourTableName]
GROUP BY [Name]/[Any other field you would like your last records to appear by]
You should now have a table containing the Names in one column and the last available ID for that Name.
Now you can use a join to get the other details from your primary table, say this is some price or date then run the following:
SELECT a.*,b.Price/b.date/b.[Whatever other field you want]
FROM [TableName] a LEFT JOIN [YourTableName]
ON a.Name = b.Name and a.LastID = b.pID
This should then give you the last records for each Name, for the first record run the same queries as above just replace the Max by Min above.
This should be easy to follow and should run quicker as well
If you don't have any identifying columns you could use to get the insert order. You can always do it like this. But it's hacky, and not very pretty.
select
t.row1,
t.row2,
ROW_NUMBER() OVER (ORDER BY t.[count]) AS rownum from (
select
tab.row1,
tab.row2,
1 as [count]
from table tab) t
So basically you get the 'natural order' if you can call it that, and add some column with all the same data. This can be used to sort by the 'natural order', giving you an opportunity to place a row number column on the next query.
Personally, if the system you are using hasn't got a time stamp/identity column, and the current users are using the 'natural order', I would quickly add a column and use this query to create some sort of time stamp/incremental key. Rather than risking having some automation mechanism change the 'natural order', breaking the data needed.
I think this code may help you:
WITH cte_Loans
AS
(
SELECT LoanID
,[Status]
,ROW_NUMBER() OVER(ORDER BY (SELECT 1)) AS RN
FROM LoanTable
)
SELECT LoanID
,[Status]
FROM LoanTable L1
WHERE RN = ( SELECT max(RN)
FROM LoanTable L2
WHERE L2.LoanID = L1.LoanID)