I am very much a beginner and I completely get what NOT IN does, but don't really get EXISTS or NOT EXISTS.
Even more, I don't understand what this does:
SELECT TOP 1 1
FROM tblSomeTable
What does this query actually do?
For reference, I have been working with something like this:
SELECT COUNT(E_ID)
FROM tblEmployee e
INNER JOIN tblManager m
ON e.tbl_ID = m.tbl_ID
WHERE NOT EXISTS(SELECT TOP 1 1
FROM tblEmployee e2
WHERE e2.E_ID = e.E_ID
AND isFired = 'N'
)
I suppose I haven't read/seen a layman's explanation yet that makes sense to me. Even after reading Diff between Top 1 1 and Select 1 in SQL Select Query I still don't get it
The question that I think would actually need answering is whether EXISTS (SELECT TOP 1 1 FROM MyTable) is actually necessary.
Top 1 1 is telling the query to pick the constant "1" for any answer.
The Top 1 part is telling it to stop as soon as it finds a match and returns "1".
Wouldn't EXISTS (SELECT TOP 1 FROM MyTable) be sufficient?
Your first query will get you only top most record (very first record) out of the total rows in result set. So, if your query returns 10 rows .. you will get the first row. Read more about TOP
SELECT TOP 1 FROM tblSomeTable
In your Second query the part under () is a subquery, in your case it's a correlated subquery which will be evaluated once for each row processed by the outer query.
NOT EXISTS will actually check for existence of the rows present in subquery
WHERE NOT EXISTS
(
SELECT TOP 1 1 FROM tblEmployee e2 WHERE e2.E_ID = e.E_ID AND isFired = 'N'
)
Read more about Correlated subquery as well as Subqueries with EXISTS
SELECT TOP 1 1 FROM <table> will return you the first row with the value as 1 always, which you have defined as constant.
So if you change this to SELECT TOP 1 2 FROM <table> it will return the value as 2 always.
Difference between IN and EXISTS operators in SQL
Please read this:
http://awesomesql.wordpress.com/2009/07/31/difference-between-in-and-exists-operators-in-sql/
Related
hi i have 3 questions on sql please :
1-about this simple code
1. with cte as (
2. select * from TABLE1)
3. select * from cte
when select * from TABLE1 compute?
first the line 3 call and then line 1 then line 2
or first 1+2 and then 3?
2- when i do left/right join i have got some row with null, that make sense .
but how can i insert to the row will null "0" instead null?
to all of the row that because the left/right join get null ( if i use inner join i will not show this row )
thanks!
The order of execution is up to the database. The order of execution will depend on tables statistics and other factors. I've seen both order of execution.
If you have a NULL value and you want to show zero, use NVL for Oracle, e.g. NVL(myColumn,0) this will return myColumn if it's not null, otherwise 0. ISNULL for SQL Server and MySQL.
Please find below image for make understanding my issues. I have a table as shown below picture. I need to get only highlighted (yellow) records. What is the best method to find these records?
In SQL Server 2012+, you can use the lead() and lag() functions. However, this is not available in SQL Server 2008. Here is a method using outer apply:
select t.*
from t outer apply
(select top 1 tprev.*
from t tprev
on tprev.time < t.time
order by tprev.time desc
) tprev outer apply
(select top 1 tnext.*
from t tnext
on tnext.time > t.time
order by tnext.time asc
)
where (t.cardtype = 1 and tnext.cardtype = 2) or
(t.cardtype = 2 and tprev.cardtype = 1);
With your sample data, it would also be possible to use self joins on the id column. This seems unsafe, though, because there could be gaps in that columns values.
Havent tried this, but I think it should work. First, make a view of the table in your question, with the rownumber included as one column:
CREATE VIEW v AS
SELECT
ROW_NUMBER() OVER(ORDER BY id) AS rownum,
id,
time,
card,
card_type
FROM table
Then, you can get all the rows of type 1 followed by a row of type 2 like this:
SELECT
a.id,
-- And so on...
FROM v AS a
JOIN v AS b ON b.rownum = a.rownum + 1
WHERE a.card_type = 1 AND b.card_type = 2
And all the rows of type 2 preceded by a row of type 1 like this:
SELECT
b.id,
-- And so on...
FROM v AS b
JOIN v AS a ON b.rownum = a.rownum + 1
WHERE a.card_type = 1 AND b.card_type = 2
To get them both in the same set of results, you can just use UNION ALL. Technically, you don't need the view. You could use nested selects instead, but since you will need to query the table four times it might be nice to have it as a view.
Also, if the ID is continous (it goes 1, 2, 3 without any gaps), you don't need the rownum and can just use the ID instead.
here is a code you can run in sql server
select * from Table_name where id in (1,2,6,7,195,160,164,165)
Supposed you have a table T(A) with only positive integers allowed, like:
1,1,2,3,4,5,6,7,8,9,11,12,13,14,15,16,17,18
In the above example, the result is 10. We always can use ORDER BY and DISTINCT to sort and remove duplicates. However, to find the lowest integer not in the list, I came up with the following SQL query:
select list.x + 1
from (select x from (select distinct a as x from T order by a)) as list, T
where list.x + 1 not in T limit 1;
My idea is start a counter and 1, check if that counter is in list: if it is, return it, otherwise increment and look again. However, I have to start that counter as 1, and then increment. That query works most of the cases, by there are some corner cases like in 1. How can I accomplish that in SQL or should I go about a completely different direction to solve this problem?
Because SQL works on sets, the intermediate SELECT DISTINCT a AS x FROM t ORDER BY a is redundant.
The basic technique of looking for a gap in a column of integers is to find where the current entry plus 1 does not exist. This requires a self-join of some sort.
Your query is not far off, but I think it can be simplified to:
SELECT MIN(a) + 1
FROM t
WHERE a + 1 NOT IN (SELECT a FROM t)
The NOT IN acts as a sort of self-join. This won't produce anything from an empty table, but should be OK otherwise.
SQL Fiddle
select min(y.a) as a
from
t x
right join
(
select a + 1 as a from t
union
select 1
) y on y.a = x.a
where x.a is null
It will work even in an empty table
SELECT min(t.a) - 1
FROM t
LEFT JOIN t t1 ON t1.a = t.a - 1
WHERE t1.a IS NULL
AND t.a > 1; -- exclude 0
This finds the smallest number greater than 1, where the next-smaller number is not in the same table. That missing number is returned.
This works even for a missing 1. There are multiple answers checking in the opposite direction. All of them would fail with a missing 1.
SQL Fiddle.
You can do the following, although you may also want to define a range - in which case you might need a couple of UNIONs
SELECT x.id+1
FROM my_table x
LEFT
JOIN my_table y
ON x.id+1 = y.id
WHERE y.id IS NULL
ORDER
BY x.id LIMIT 1;
You can always create a table with all of the numbers from 1 to X and then join that table with the table you are comparing. Then just find the TOP value in your SELECT statement that isn't present in the table you are comparing
SELECT TOP 1 table_with_all_numbers.number, table_with_missing_numbers.number
FROM table_with_all_numbers
LEFT JOIN table_with_missing_numbers
ON table_with_missing_numbers.number = table_with_all_numbers.number
WHERE table_with_missing_numbers.number IS NULL
ORDER BY table_with_all_numbers.number ASC;
In SQLite 3.8.3 or later, you can use a recursive common table expression to create a counter.
Here, we stop counting when we find a value not in the table:
WITH RECURSIVE counter(c) AS (
SELECT 1
UNION ALL
SELECT c + 1 FROM counter WHERE c IN t)
SELECT max(c) FROM counter;
(This works for an empty table or a missing 1.)
This query ranks (starting from rank 1) each distinct number in ascending order and selects the lowest rank that's less than its number. If no rank is lower than its number (i.e. there are no gaps in the table) the query returns the max number + 1.
select coalesce(min(number),1) from (
select min(cnt) number
from (
select
number,
(select count(*) from (select distinct number from numbers) b where b.number <= a.number) as cnt
from (select distinct number from numbers) a
) t1 where number > cnt
union
select max(number) + 1 number from numbers
) t1
http://sqlfiddle.com/#!7/720cc/3
Just another method, using EXCEPT this time:
SELECT a + 1 AS missing FROM T
EXCEPT
SELECT a FROM T
ORDER BY missing
LIMIT 1;
Because this is part of a larger SQL SELECT statement, I want a wholly SQL query that selects the first item matching a criteria or, if no items match the criteria, just the first item.
I.e. using Linq I want:
Dim t1 = From t In Tt
Dim t2 = From t In t1 Where Criteria(t)
Dim firstIfAny = From t In If(t2.Any, t2, t1) Take 1 Select t
Because If is not part of Linq, LinqPad doesn't show a single SQL statement, but two, the second depending upon whether the Criteria matches any of the Tt values.
I know it will be SELECT TOP 1 etc. and I can add ORDER BY clauses to get the specific first one I want, but I'm having trouble thinking of the most straightforward way to get the first of two criteria. (It was at exactly this point when I was able to solve this myself.)
Seeing as I don't see an existing question for this, I will let it stand. I'm sure someone else will see the answer quickly.
select top 1 *
from (
select top 1 *, 1 as Rank from MyTable where SomeColumn = MyCriteria
union all
select top 1 *, 2 as Rank from MyTable order by MyOrderColumn
) a
order by Rank
I've gone with this:
SELECT TOP 1 *
FROM MyTable
WHERE SomeColumn = MyCriteria
OR NOT (EXISTS (SELECT NULL FROM MyTable WHERE SomeColumn = MyCriteria))
ORDER BY MyOrdering
My actual SomeColumn = MyCriteria is rather more complex of course, as well as other unrelated where clauses.
In my application I use SELECT TOP 12 * clause to select top 12 records from database and show it to user. In another case I have to show the same result one by one. So I use SELECT TOP 1 * clause,rest of the query is same. I used Sql row_number() function to select items one by on serially.
The problem is SELECT TOP 1 * doesn't return me same row as I get in SELECT TOP 12 *. Also the result set of SELECT TOP 12 * get changed each time I execute the query.
Can anybody explain me why the result is not get same in SELECT TOP 12 * and SELECT TOP 1 *.
FYI: here is my sql
select distinct top 1 * from(
select row_number() over ( ORDER BY Ratings desc ) as Row, * from(
SELECT vw.IsHide, vw.UpdateDate, vw.UserID, vw.UploadPath, vw.MediaUploadID, vw.Ratings, vw.Caption, vw.UserName, vw.BirthYear, vw.BirthDay, vw.BirthMonth, vw.Gender, vw.CityProvince, vw.Approved
FROM VW_Media as vw ,Users as u WITH(NOLOCk)
WHERE vw.IsHide='false' and
GenderNVID=5 and
vw.UserID=u.UserID and
vw.UserID not in(205092) and
vw.UploadTypeNVID=1106 and
vw.IsDeleted='false' and
vw.Approved = 1 and
u.HideProfile=0 and
u.StatusNVID=126 and
vw.UserID not in(Select BlockedToUserID from BlockList WITH(NOLOCk) where UserID=205092) a) totalres where row >0
Thanks in Advance
Sachin
When you use SELECT TOP, you must use also the ORDER BY clause to avoid different results every time.
For performance resons, the database is free to return the records in any order it likes if you don't specify any ordering.
So, you always have to specify in which order you want the records, if you want them in any specific order.
Up to some version of SQL Server (7 IIRC) the natural order of the table was preserved in the result if you didn't specify any ordering, but this feature was removed in later versions.