This sqlite query returns zero rows as expected:
SELECT 1 FROM tbl WHERE 0;
This query, on the other hand, returns one row containing a null column:
SELECT MAX(1) FROM tbl WHERE 0;
Why does the second query return one row, rather than zero rows?
'Normal' queries return one result for each (filtered) record of the table in the FROM clause.
However, when you are using some aggregate function, the result has one record for each group in the source table. And if there is no GROUP BY clause, the entire table is one group.
With GROUP BY, there are no results for empty groups.
However, without GROUP BY, you always have exactly one group and get one result record, even if that group ends up being empty.
Your query would get zero rows with MAX if it were also using GROUP BY (with a constant value, which would put all records, if there were any, into a single group):
SELECT MAX(1) FROM tbl WHERE 0 GROUP BY NULL;
Related
I have a table where rows appear to be "duplicates" but they are actually not (they have different date).
Suppose each record has a column A that is supposed to be unique. However due to this column A could or could not appear again later with updated information (with column A unchanged), it is no longer unique even when it should be.
Therefore I want the table with latest information only. Currently this table contains 500k entries, however the "true" number of unique entries is less than half of it.
I have tried
SELECT *
FROM TABLE
WHERE A = A
AND Date = (SELECT MAX(Date) from TABLE)
ORDER BY DATE
However this only returns 2 results. How do I achieve that?
The subquery on the date is the correct idea, but you must include the column A in the subquery and relate it back to the main table. I prefer to use explicit joins rather than embedding the subquery in the WHERE statement. This is usually more efficient anyway.
SELECT TABLE.*
FROM TABLE INNER JOIN
(SELECT A, MAX(Date) AS MaxDate FROM TABLE GROUP BY A) AS latest
ON TABLE.A = latest.A AND TABLE.date = latest.MaxDate
ORDER BY A, date
Or even better, I prefer CTE (Common Table Expression) syntax, since it makes the individual queries easier to read:
WITH latest AS (
SELECT A, MAX(Date) AS MaxDate
FROM TABLE
GROUP BY A
)
SELECT TABLE.*
FROM TABLE INNER JOIN latest
ON TABLE.A = latest.A AND TABLE.date = latest.MaxDate
ORDER BY TABLE.A, TABLE.date
Comparison to other answer
The answer by MikeT relies on a non-standard feature of sqlite. That is okay of itself as long as you are aware that the solution is not compatible with other databases engines/servers and SQL dialects.
The next possible gotcha really relies on your actual data and table schema (neither of which you shared in the question details). If your data allows multiple rows with the same date for the a single A column value, then the conditions in your question are not enough to definitively remove all duplicates. You would need to identify another column by which to resolve any remaining duplicates, but once again your question did not do that.
However, in testing, I found that my solution allows unresolved duplicates to remain in the results. MikeT's solution eliminate all duplicates, but it does so by arbitrarily excluding one of those duplicates. There are ways to fix either solution to definitely select which duplicate to keep, but I will not even attempt that unless you post actual data and the table schema so that my answer is not just mere guessing. I'm glad that my answer was useful thus far, but you need to understand your data better (than reveal in the question) to ensure what solution is actually best.
Bonus
Against my better judgement to just keep expanding on answers... since you should really research this separately... here's an example of how you would continue joining this with other queries...
WITH latest AS (
SELECT A, MAX(Date) AS MaxDate
FROM TABLE
GROUP BY A
),
firstResults AS (
SELECT TABLE.*
FROM TABLE INNER JOIN latest
ON TABLE.A = latest.A AND TABLE.date = latest.MaxDate
ORDER BY TABLE.A, TABLE.date
)
SELECT otherTable.*
FROM firstResults JOIN otherTable
ON firstResults.A = otherTable.A
WHERE somecondition = 'foobar'
Another approach if you're using a somewhat recent version of sqlite (3.25 or newer), using the row_number() window function to rank groups of the same a value by date and picking the first one:
WITH cte AS
(SELECT a, date, row_number() OVER (PARTITION BY a ORDER BY date DESC) AS rn
FROM yourtable)
SELECT a, date
FROM cte
WHERE rn = 1;
One important thing to note since I noticed you mentioning another answer was slow is that an index on mytable(a, date DESC) will be needed for this query for best results, and an index on mytable(a, date) will speed up the other answers given.
I believe, if I understand what you have written, that you could use :-
SELECT a,max(date), other FROM mytable GROUP BY a ORDER BY date;
note that the other column represents other columns (if present)
However, the other column will be an arbritary value (from one of the grouped columns) which may well be the required value (in the example it is).
As per :-
Each expression in the result-set is then evaluated once for each
group of rows. If the expression is an aggregate expression, it is
evaluated across all rows in the group. Otherwise, it is evaluated
against a single arbitrarily chosen row from within the group. If
there is more than one non-aggregate expression in the result-set,
then all such expressions are evaluated for the same row.
SQL As Understood By SQLite - SELECT
More correctly, to eliminate an arbritary value(sic) for the other column, you could use :-
SELECT
a /* will always be the same and isn't arbritary */,
max(date) /* will be the maximum data */ AS date,
(SELECT other FROM mytable WHERE a = m.a AND date = m.date) AS other
FROM mytable AS m /* AS m allows the outer query to be distinguished from the inner query */
GROUP BY a /* this effectivel removes duplicates on the a column */
ORDER BY date
;
The example below appears to produce the same result.
Example :-
Using the following to populate the table with some generated testing data :-
CREATE TABLE IF NOT EXISTS mytable (a TEXT, date TEXT, other);
WITH cte(count,a,date,other) AS
(
SELECT 1,1,date('now','+'||(random() % 30)||' days'),'other1'
UNION ALL SELECT count+1,abs(random()) % 20,date('now','+'||(abs(random()) % 30)||' days'), 'other'||(count+1) FROM cte LIMIT 100
INSERT INTO mytable (a,date,other) SELECT a,date,other FROM cte
;
SELECT * FROM mytable ORDER BY DATE DESC;
in this case :-
Highlighted rows being those required to be extracted.
Then after the above has been run the following is run
SELECT * FROM mytable WHERE a = a AND date = (SELECT MAX(date) FROM mytable);
SELECT * FROM mytable WHERE /*a = a AND*/ date = (SELECT MAX(date) FROM mytable);
/* Will only select 1 row per unique value of a BUT other will be an arbritary value not necessairlly the latest */
SELECT a,max(date), other FROM mytable GROUP BY a /* group by effectively display unique */;
SELECT
a /* will always be the same and isn't arbritary */,
max(date) /* will be the maximum data */ AS date,
(SELECT other FROM mytable WHERE a = m.a AND date = m.date) AS other
FROM mytable AS m
GROUP BY a
;
The first two results show that a = a does nothing as it will always be true.
The thrid query produces (unordered) :-
Note ticks assigned by checking the value of other from the previous result.
In this case this shorter query works OK even though values of other are arbritary values (they aren't really as it depends upon how the query planner plasn the query).
The fourth, the more correct, produces the same results :-
Result 2 (your orignal query) and 3 (original without a = a) produce :-
and :-
Can someone please explain me, what is the difference between qualify...over...partition by and group by...having in Teradata?I would also like to know if there are any differences in their performances.
QUALIFY is a proprietary extension to filter the result of a Windowed Aggregate Function.
A query is logically processed in a specific order:
FROM: create the basic result set
WHERE: remove rows from the previous result set
GROUP BY: apply aggregate functions on the previous result set
HAVING: remove rows from the previous result set
OVER: apply windowed aggregate functions on the previous result set
QUALIFY: remove rows from the previous result set
Having clause is used to filter the result set of the aggregate functions like (COUNT,min,max etc)
they eliminate rows based from groups based on some criteria like this :-
SELECT dept_no, MIN(salary), MAX(salary), AVG(salary)
FROM employee
WHERE dept_no IN (100,300,500,600)
GROUP BY dept_no
HAVING AVG(salary) > 37000;
The QUALIFY clause eliminates rows based on the function value, returning a new value for each of the participating rows.
It works on the final result set.
SELECT NAME,LOCATION FROM EMPLOYEE
QUALIFY ROW_NUMBER() OVER ( PARTITION BY NAME ORDER BY JOINING_DATE DESC) = 1;
We can club both having and qualify as well in a query if we use both aggregate and analytical fucntion like below:-
SELECT StoreID, SUM(sale),
SUM(profit) OVER (PARTITION BY StoreID)
FROM facts
GROUP BY StoreID, sale, profit
HAVING SUM(sale) > 15
QUALIFY SUM(profit) OVER (PARTITION BY StoreID) > 2;
You can see there order of execution from dnoeth answer.
I want to exclude entire rows that have blanks for a specific column....I dont want to show a row that has the first name column as a blank...I was thinking maybe a case statement would help with this
case when [First Name] = blank then exclude???
If this is SQL, in general you could do this:
SELECT *
FROM myTable
WHERE myCol IS NOT NULL
I had to add a calculated column that would be used as a flag, and then filter on the flag.
if(Len([First Name])>0,1,0)
Now I am able to filter on the calculated column to show only rows that have 1 in it (which will be the rows where First Name is not empty).
This doesn't delete the row from the dataset, but has the same effect in visualizations as if it had been.
Why query:
SELECT id, MAX(probe_time) AS Expr1
FROM app_states
GROUP BY logon_id
and
SELECT id, MIN(probe_time) AS Expr1
FROM app_states
GROUP BY logon_id
has same id result.
I wish to select row with MAX or MIN time for every user.
I'm afraid you'll have to use sub queries here.
SELECT id, probe_time
FROM app_states
WHERE probe_time = (SELECT MAX(probe_time) from app_states GROUP BY logon_id)
BTW, SQLite does interesting optimization on MIN/MAX:
Queries of the following forms will be optimized to run in logarithmic time assuming appropriate indices exist:
SELECT MIN(x) FROM table;
SELECT MAX(x) FROM table;
In order for these optimizations to occur, they must appear in exactly the form shown above - changing only the name of the table and column. It is not permissible to add a WHERE clause or do any arithmetic on the result. The result set must contain a single column. The column in the MIN or MAX function must be an indexed column.
— http://www.sqlite.org/optoverview.html#minmax
I have a table and for the sake of the example lets say it has three columns: Name, DateAdded, Note. In this table there will be multiple records with the same name. I need a linq where clause that will get me a result set with the most recent record for each name.
So if I had the following information:
Name, DateAdded, Note
Alpha, 1/1/2010, note one
Alpha, 1/2/2010, note two
Alpha, 1/3/2010, note three
Beta, 1/4/2010, note four
Beta, 1/5/2010, note five
I would expect to get the following results:
Name, DateAdded, Note
Alpha, 1/3/2010, note three
Beta, 1/5/2010, note five
In real use this is going to be a search option so I need it to be a .Where clause I can append to an IQueryable collection if possible.
from row in rows
group row by row.Name into grouping
from g in grouping
where g.DateAdded == grouping.Max(x => x.DateAdded)
select g;
If there's more than one row with the maximum DateAdded in some group, this returns all such rows.
You don't need just one Where clause. If rows is a IQueryable, then the result of this query is too, so you can use it as a part of a chain. But I have no idea how well does this query translate to SQL.