select 1 from MY_TABLE where ID=42 fetch first 1 rows only;
select 1 from MY_TABLE where ID=42 and rownum=1;
select case when exists (select 1 from MY_TABLE where ID=42) then 1 else 0 end from dual;
select count(1) from MY_TABLE where ID=42;
Here is 4 methods to check whether the table contains any rows. The last one is quite slow, but first 3 has approximatively the same time. What is the best practice (academically correct)?
Any other methods?
Unless throwing an exception is acceptable, in which case you will need to capture it, you will want to use a count with rownum = 1
select count(*) from my_table where id = 42 and rownum = 1;
In this case, the return value of 0 means there are no rows, 1 means there are 1 or more rows.
While this gives the same results as your select with case statement, it is much less complicated and easier to understand.
Related
Let's say I have a table with two columns:
| ID | A |
I want to sort by A, then get the 10 records after a given ID. What would be the best way to handle this in Postgres?
To clarify, I want to sort by A, but do my pagination by the ID. So if I had a table like:
1 | 'C'
2 | 'B'
3 | 'A'
4 | 'G'
5 | 'A'
6 | 'H'
So after sorting by A, I'd want the first three values after id=1, so:
1 | 'C'
4 | 'G'
6 | 'H'
An ordering of any column is purely dependant on the order by clause and the terms "before" or "after" come into picture only when there's a pre-determined order. So, once the records are ordered by column "A", there's no guarantee that the id's will be ordered in the sequence 1,4,6, unless you also specified that ordering of id.
So, if you
want the first three values after id=1
It means there should be a way to determine the point where the id value has become 1 and all the rows beyond are to be considered. To ensure that you have to explicitly include id in the order by. A COUNT analytic function can come to our rescue to mark the point.
SELECT id,a
FROM ( SELECT t.*,COUNT(CASE WHEN id = 1 THEN 1 END) --the :id argument
OVER( ORDER BY a,id) se
FROM t order by a,id --the rows are ordered first by a, then by id
-- same as in the above count analytic function
) s
WHERE se = 1 limit 3; -- the argument 3 or 10 that you wish to pass
-- se = 1 won't change for other ids, it's a marker
-- that id = n is reached
DEMO
I think this will do:
SELECT *
FROM (SELECT * from MyTable where ID > givenId order by A) sub
LIMIT 10;
You don't want the A columns, so:
SELECT r.*
FROM t
WHERE t.id > ANY (SELET id FROM t t2 WHERE t2.col = 'A')
ORDER BY col
LIMIT 10;
Note that this does not return any rows with A as the value. It also works when the comparison value is not sorted first.
this will work:
SELECT * from Table1 where "ID"=1
order by "A" desc limit 2;
check :http://sqlfiddle.com/#!15/5854b/3
for your query :
SELECT * from Table1 where "ID"=1
order by "A" desc limit 10;
I have this
RECORD ITEMS ITEMSTOTAL
------------------------------------ ---------- ----------
ababababaa 0 1
ababababab 0 0
ababababac 0 1
ababababad 1 1
ababababae 0 2
but I need this output when ITEMS=ITEMSTOTAL
RECORD
------------------------------------
ababababab
ababababad
Currently I'm using this query for the first result, but I don't know how to get the second output, Maybe this sounds obvious but I can't find the answer :(
SELECT RECORD,
(SELECT COUNT(*) FROM TABLE1 WHERE SOMETHING=X) AS ITEMS,
(SELECT COUNT(*) FROM TABLE2 WHERE SOMETHING2=Y) AS ITEMSTOTAL
FROM RECORDS_TABLE
WHERE DELETED=0
--and ITEMS.count = ITEMSTOTAL.count <-- tried something like this but it doesn't work.
One option would be to use a subquery and apply the where criteria to the outer query.
select *
from (
your query here
) t
where items = itemstotal
I assume that isn't your actual query btw. where comes after from. Also, those count statements would return the same values.
With the sql below I count how many records I have in tableB for each code. The total field is assigned the result of the count and the code the code field of the record.
SELECT
"count" (*) as total,
tableB."code" as code
FROM
tableB
WHERE
tableB.code LIKE '%1'
GROUP BY
tableB.code
In tableA I have a sequence field and I update with the result of total (obtained in the previous sql) plus 1 Do this for each code.
I tried this and it did not work, can someone help me?
UPDATE tableA
SET tableA.sequence = (tableB.total + 1) where tableA."code" = tableB.code
FROM
(
SELECT
"count" (*) as total,
tableB."code" as code
FROM
tableB
WHERE
tableB.code LIKE '%1'
GROUP BY
tableB.code
)
I edited for my tables are as mostar believe facillita understanding of my need
tableA
code sequence
100 null
200 null
table B
code sequence
100 1
100 2
100 3
100 4
......
100 17
200 1
200 2
200 3
200 4
......
200 23
Need to update the sequence blank field in tableA with the number 18 to code = 100
Need to update the sequence blank field in tableA with the number 24 to code = 200
This assumes that code is unique in table_a:
with max_seq as (
select code,
max(sequence) + 1 as max_seq
from table_b
group by code
)
update table_a
set sequence = ms.max_seq
from max_seq ms
where table_a.code = ms.code;
SQLFiddle example: http://sqlfiddle.com/#!15/745a7/1
UPDATE tbl_a a
SET sequence = b.next_seq
FROM (
SELECT code, max(sequence) + 1 AS next_seq
FROM tbl_b
GROUP BY code
) b
WHERE a.code = b.code;
SQL Fiddle.
Only columns of the target table can be updated. It would not make sense to table-qualify those. Consequently, this is not allowed.
Every subquery must have a table alias for the derived table.
I would not use a CTE for a simple UPDATE like this. A subquery in the FROM clause is typically simpler and faster.
No point in double-quoting the aggregate function count(). No pint in double-quoting perfectly legal, lower case identifiers, either. No point in table-qualifying columns in a subquery on a single table in a plain SQL command (no harm either).
You don't need a WHERE condition, since you want to UPDATE all rows (as it seems). Note that only row with matching code are updated. Other rows in tbl_b remain untouched.
Basically you need to read the manual about UPDATE before you try any of this.
I am not able to understand the SQL query output :
SQL> select distinct(STATUS) from TMP_ORDER_ACTION_PSTN_CP_11035;
InDelivery_SOMBe
In Delivery
Complete
Amended
Cancelled
Failed InComplete
1 SQL> select count(*) from TMP_ORDER_ACTION_PSTN_CP_11035 where
STATUS='Complete';
1484
2 SQL> select count(*) from TMP_ORDER_ACTION_PSTN_CP_11035 where STATUS
!= 'Complete';
3167
3 SQL> select count(*) from TMP_ORDER_ACTION_PSTN_CP_11035;
5091
The sum of count for the 1 and 2 queries should be same as the total count(3 query).Why is the sum differing from the whole count?
It seems like a dump question but i dont know why is this happening.
Please note that My question is not related to null check at all.It is the that
sum(1+2)=3.1484+3167 !=5091.Why is the result different?
My guess is NULL values, which match none of your WHERE clauses, including the last one. Try
select count(*) from TMP_ORDER_ACTION_PSTN_CP_11035 where STATUS is null;
where status = null is never true, nor is where null = null. You have to use is null.
The sum of count for the 1 and 2 queries should be same as the total count(3 query).Why is the sum differing from the whole count?
No, because the records with NULL are not matching query 1 or query 2, but they are counted in query 3.
1 + 2 + IS NULL should equal 3.
WHERE STATUS = NULL won't work. Nothing equals NULL.
Try IS instead of =...
select count(*) from TMP_ORDER_ACTION_PSTN_CP_11035 where STATUS IS null
Try this:
Assuming p_key is a primary key for the table,
select count(p_key) from TMP_ORDER_ACTION_PSTN_CP_11035 where STATUS='Complete';
select count(p_key) from TMP_ORDER_ACTION_PSTN_CP_11035 where STATUS <> 'Complete';
select count(p_key) from TMP_ORDER_ACTION_PSTN_CP_11035 ;
Given a table (mytable) containing a numeric field (mynum), how would one go about writing an SQL query which summarizes the table's data based on ranges of values in that field rather than each distinct value?
For the sake of a more concrete example, let's make it intervals of 3 and just "summarize" with a count(*), such that the results tell the number of rows where mynum is 0-2.99, the number of rows where it's 3-5.99, where it's 6-8.99, etc.
The idea is to compute some function of the field that has constant value within each group you want:
select count(*), round(mynum/3.0) foo from mytable group by foo;
I do not know if this is applicable to mySql, anyway in SQL Server I think you can "simply" use group by in both the select list AND the group by list.
Something like:
select
CASE
WHEN id <= 20 THEN 'lessthan20'
WHEN id > 20 and id <= 30 THEN '20and30' ELSE 'morethan30' END,
count(*)
from Profiles
where 1=1
group by
CASE
WHEN id <= 20 THEN 'lessthan20'
WHEN id > 20 and id <= 30 THEN '20and30' ELSE 'morethan30' END
returns something like
column1 column2
---------- ----------
20and30 3
lessthan20 3
morethan30 13