A co-worker just came to me with a puzzling SQL query:
(essentially)
SELECT LEAST(id) FROM tableA A, tableB B WHERE a.name = b.name(+)
The result set returned lists three numbers however:
LEAST(id)
--------------
621
644
689
(all being IDs that meet the query as if it lacked the LEAST function all together)
Why? =)
LEAST(x,y,...) is not an aggregate function. It works only on its parameters. The function you want is MIN(x).
For each record, you're running LEAST(id), which will always return id. If you were passing LEAST more parameters, you would see different results. For example, LEAST(5,6,7) = 5. LEAST always returns the smallest of its parameters, whereas MIN returns the smallest of every record.
Related
what is the difference between the select count(*) and select count(true)?
so is there any different between the count(*) and count(true) which one should I use?
can you give me situation example for each one that is better option to choose?
The result of both is the same, but count(*) is slightly faster than count(true). That is because in the first case, the aggregate function has no arguments (that's what the * means in SQL), whereas in the second case the argument true is checked for NULLness, since count skips rows where the argument is NULL.
The same result, it will give you total number of rows in a table
What does this query try to achieve?
SELECT * FROM X WHERE (X.Y in (select Y from X))
As far as I figured, it is yielding me the same result as
SELECT * FROM X WHERE Y is not NULL
Is there anything more to the first query? The first query is actually very slow with a large dataset and hence I want to know whether I can replace it with the second query.
You are right, the two queries are equivalent.
It is unclear, why the first query was written this way. Maybe it looked different once.
As is, your second query is better, because it is easier to read and understand (and even faster as you say).
your second query is perfect than the 1st one
because in 1st query you may get abnormal(null) result in case if column Y contains null value but you will not get abnormal result in 2nd one if null values contain in column Y.
So based on values of your table two query will behave two different way
I am getting very strange behavior on 2.0-M2. Consider the following against the GratefulDeadConcerts database:
Query 1
SELECT name, in('written_by') AS wrote FROM V WHERE type='artist'
This query returns a list of artists and the songs each has written; a majority of the rows have at least one song.
Query 2
Now try:
SELECT name, count(in('written_by')) AS num_wrote FROM V WHERE type='artist'
On my system (OSX Yosemite; Orient 2.0-M2), I see just one row:
name num_wrote
---------------------------
Willie_Cobb 224
This seems wrong. But I tried to better understand. Perhaps the count() causes the in() to look at all written_by edges...
Query 3
SELECT name, in('written_by') FROM V WHERE type='artist' GROUP BY name
Produces results similar to the first query.
Query 4
Now try count()
SELECT name, count(in('written_by')) FROM V WHERE type='artist' GROUP BY name
Wrong path -- So try LET variables...
Query 5
SELECT name, $wblist, $wbcount FROM V
LET $wblist = in('written_by'),
$wbcount = count($wblist)
WHERE type='artist'
Produces seemingly meaningless results:
You can see that the $wblist and $wbcount columns are inconsistent with one another, and the $wbcount values don't show any obvious progression like a cumulative result.
Note that the strange behavior is not limited to count(). For example, first() does similarly odd things.
count(), like in RDBMS, computes the sum of all the records in only one value. For your purpose .size()seems the right method to call:
in('written_by').size()
I just stumbled over jOOQ's maxDistinct SQL aggregation function.
What does MAX(DISTINCT x) do different from just MAX(x) ?
maxDistinct and minDistinct were defined in order to keep consistency with the other aggregate functions where having a distinct option actually makes a difference (e.g., countDistinct, sumDistinct).
Since the maximum (or minimum) calculated between the distinct values of a dataset is mathematically equivalent with the simple maximum (or minimum) of the same set, these function are essentially redundant.
In short, there will be no difference. In case of MySQL, it's even stated in manual page:
Returns the maximum value of expr. MAX() may take a string argument;
in such cases, it returns the maximum string value. See Section 8.5.3,
“How MySQL Uses Indexes”. The DISTINCT keyword can be used to find the
maximum of the distinct values of expr, however, this produces the
same result as omitting DISTINCT.
The reason why it's possible - is because to keep compatibility with other platforms. Internally, there will be no difference - MySQL will just omit influence of DISTINCT. It will not try to do something with set of rows (i.e. produce distinct set first). For indexed columns it will be Select tables optimized away (thus reading one value from index, not a table), for non-indexed - full scan.
If i'm not wrong there are no difference
For Columns
ID
1
2
2
3
3
4
5
5
The OUTPUT for both quires are same 5
MAX(DISTINCT x)
// ID = 1,2,2,3,3,4,5,5
// DISTINCT = 1,2,3,4,5
// MAX = 5
// 1 row
and for
MAX(x)
// ID = 1,2,2,3,3,4,5,5
// MAX = 5
// 1 row
Theoretically, DISTINCT x ensures that every element is different from a certain set. The max operator selects the highest value from a set. In plain SQL there should be no difference between both.
what am I doing wrong with my sql query? It always return an empty rows even if there is a value exist.
Here is my query:
SELECT *
FROM users
WHERE user_theme_id IN ( 9735, 9325, 4128 )
AND ( user_date_created BETWEEN '2013-06-04' AND '2013-06-10' );
I tried to cut my original query one by one, I got a result. Here is the first one:
SELECT * FROM users WHERE user_theme_id IN (9735, 9325, 4128 );
I got 3 rows for this result. See attached snapshot:
Now, the next query that I run is this:
SELECT *
FROM users
WHERE user_date_created BETWEEN '2013-06-04' AND '2013-06-10';
I do get 3 results on this. See attached snapshot:
By the way, this sql that uses BETWEEN should suppose return 4 rows but it only return 3. It doesn't return the data which has the created date of 2013-06-10 08:27:43
What am I doing wrong with my original query Why does it always return an empty rows?
If you are getting results by separately running different where clauses doesn't guarantee that AND 2 where clauses will return an answer.
There has to be intersection of rows to get result while AND.
You should validate your data and see if overlapping exists.
I have able to make it work by not using the SQL BETWEEN operators but instead COMPARISON OPERATORS like: >= || <=
I have read it from W3schools.com, the SQL between can produce different results in different databases.
This is the content:
Notice that the BETWEEN operator can produce different result in different databases!
In some databases, BETWEEN selects fields that are between and excluding the test values.
In other databases, BETWEEN selects fields that are between and including the test values.
And in other databases, BETWEEN selects fields between the test values, including the first test value and excluding the last test value.
Therefore: Check how your database treats the BETWEEN operator!
That is what happened in the issue that I am facing. The first field was being treated as part of the test values and the 2nd field was being excluded. Using the comparison operators give accurate result.