I need to write a query that does this:
SELECT TOP 1
FROM a list of tables (Joins, etc)
ORDER BY Column X, Column Y, Column Z
If ColumnX is NOT NULL, then at the moment, I reselect, using a slightly different ORDER BY.
So, I do the same query, twice. If the first one has a NULL in a certain column, I return that row from my procedure. However, if the value isn't NULL - I have to do another identical select, except, order by a different column or two.
What I do now is select it into a temp table the first time. Then check the value of the column. If it's OK, return the temp table, else, redo the select and return that result set.
More details:
In english, the question I am asking the database:
Return my all the results for certain court appearance (By indexed foreign key). I expect around 1000 rows. Order it by the date of the appearance (column, not indexed, nullable), last appearance first. Check an 'importId'. If the import ID is not NULL for that top 1 row, then we need to run the same query - but this time, order by the Import ID (Last one first), and return that row. Or else, just return the top 1 row from the original query.
I'd say the BEST way to do this is in a single query is a CASE statement...
SELECT TOP 1 FROM ... ORDER BY
(CASE WHEN column1 IS NULL THEN column2 ELSE column1 END)
You could use a COALESCE function to turn nullable columns into orderby friendly values.
SELECT CAST(COALESCE(MyColumn, 0) AS money) AS Column1
FROM MyTable
ORDER BY Column1;
I used in Firebird (columns are numeric):
ORDER BY CASE <condition> WHEN <value> THEN <column1>*1000 + <column2> ELSE <column3>*1000 + <column4> END
Related
in Oracle Live SQL i was trying to use simple order by sql using select (case when) query
i tried to get to same result select * from tt order by 1
replace 1 with (select (case when 1=1 then 1 else 2 end) from dual)
but two result completely different.
i want table ordered by column 1 however the query using select case when query doesn't sort by column 1.
I don't know why and want to know how this query works in oracle db
Compare
...
order by 2
and
...
order by 1+1
At "compile" time the first 2 is an integer constant so it is a position of the column, the db engine sorts by the specified column. The second 1+1 is an integer expression and the db engine sorts by this value '2'. Same, (select (case when 1=1 then 1 else 2 end) from dual) is an expression, not a column specification.
When you specify a number in the ORDER BY clause, Oracle will sort by that column of the resulting select. As an example, ORDER BY 1,2 will sort by the first column, then the second column. If there is no second column, then you will get an error.
In the ORDER BY of the outermost query, there is essentially no sorting happening in your query because 1 is always returned from your subquery. This is sorting by the value 1 and not the first column.
If you explain the logic you are hoping to achieve, then we may be able to assist, but that is what is happening with your existing queries.
I have tried some of the various solutions posted on Stack for this issue but none of them keep null values (and it seems like the entire query is built off that assumption).
I have a table with 1 million rows. There are 10 columns. The first column is the id. Each id is unique to "item" (in my case a sales order) but has multiple rows. Each row is either completely null or has a single value in one of the columns. No two rows with the same ID have data for the same column. I need to merge these multiple rows into a single row based on the ID. However, I need to keep the null values. If the first column is null in all rows I need to keep that in the final data.
Can someone please help me with this query I've been stuck on it for 2 hours now.
id - Age - firstname - lastname
1 13 null null
1 null chris null
should output
1 13 chris null
It sounds like you want an aggregation query:
select id, max(col1) as col1, max(col2) as col2, . . .
from t
group by id;
If all values are NULL, then this will produce NULL. If one of the rows (for an id) has a value, then this will produce that value.
select id, max(col1), max(col2).. etc
from mytable
group by id
As some others have mentioned, you should use an aggregation query to achieve this.
select t1.id, max(t1.col1), max(t1.col2)
from tableone t1
group by t1.id
This should return nulls. If you're having issues handling your nulls, maybe implement some logic using ISNULL(). Make sure your data fields really are nulls and not empty strings.
If nulls aren't being returned, check to make sure that EVERY single row that has a particular ID has ONLY nulls. If one of them returns an empty string, then yes, it will drop the null and return anything else over the null.
I'd like to query the database as to whether or not one or more rows exist that satisfy a given predicate. However, I am not interested in the distinction between there being one such row, two rows or a million - just if there are 'zero' or 'one or more'. And I do not want Postgres to waste time producing an exact count that I do not need.
In DB2, I would do it like this:
SELECT 1 FROM SYSIBM.SYSDUMMY1 WHERE EXISTS
(SELECT 1 FROM REAL_TABLE WHERE COLUMN = 'VALUE')
and then checking if zero rows or one row was returned from the query.
But Postgres has no dummy table available, so what is the best option?
If I create a one-row dummy table myself and use that in place of SYSIBM.SYSDUMMY1, will the query optimizer be smart enough to not actually read that table when running the query, and otherwise 'do the right thing'?
PostgreSQL doesn't have a dummy table because you don't need one.
SELECT 1 WHERE EXISTS
(SELECT 1 FROM REAL_TABLE WHERE COLUMN = 'VALUE')
Alternatively if you want a true/false answer:
SELECT EXISTS(SELECT 1 FROM REAL_TABLE WHERE COLUMN = 'VALUE')
How about just doing this?
SELECT (CASE WHEN EXISTS (SELECT 1 FROM REAL_TABLE WHERE COLUMN = 'VALUE') THEN 1 ELSE 0 END)
1 means there is a value. 0 means no value.
This will always return one row.
If you are happy with "no row" if no row matches, you can even just:
SELECT 1 FROM real_table WHERE column = 'VALUE' LIMIT 1;
Performance is basically the same as with EXISTS. Key to performance for big tables is a matching index.
I am trying to put together a query that will display one specific record (found by the record's primary ID) at the top, and display all other records below it, sorted by date (I have "date_added" as one of the fields in the table, in addition to primary ID).
I could do this with a UNION (first select would locate the record I want, and the other select would display all other records), but I'm wondering if is there perhaps a better way?
I'm using Oracle, by the way.
You can do this by sorting by two fields
The first would be an expression that returns 0 if the row is the one you want or 1 if it isn't. Sort will be ascending so you get your preferred record first.
The second sort field would be date_added so the remaining records are sorted in this order.
Afraid I don't know oracle by in sql server it would be something like
select *
from the_table
order by (case id when 999 then 0 else 1 end), date_added desc
An easier way would be a fancy order by construct. Here's an example for pk = 123:
select *
from YourTable
order by case when yourpk = 123 then 1 else 2 end, date_added
I don't know Oracle exactly, but you could perhaps do something like..
ORDER BY IF(id == THE_ID, 0, 1), date_added
You can sort more than one record to the top using the same technique
999 first
998 second
followed by everything else sorted by date
select *
from the_table
order by (case id when 999 then 0 when 998 then 1 else 2 end), date_added desc
SELECT *
FROM `Table`
ORDER BY (`id` = THE_ID) DESC, `date_added` DESC
The simple way would be to recognise that you want to display two separate things and therefore write to separate straightforward queries. One query to retrieve the first record, and the second to retrieve the sorted list. There is no real performance advantage to doing anything more than this because of one unique record.
What is the most efficient way to select the first and last element only, from a column in SQLite?
The first and last element from a row?
SELECT column1, columnN
FROM mytable;
I think you must mean the first and last element from a column:
SELECT MIN(column1) AS First,
MAX(column1) AS Last
FROM mytable;
See http://www.sqlite.org/lang_aggfunc.html for MIN() and MAX().
I'm using First and Last as column aliases.
if it's just one column:
SELECT min(column) as first, max(column) as last FROM table
if you want to select whole row:
SELECT 'first',* FROM table ORDER BY column DESC LIMIT 1
UNION
SELECT 'last',* FROM table ORDER BY column ASC LIMIT 1
The most efficient way would be to know what those fields were called and simply select them.
SELECT `first_field`, `last_field` FROM `table`;
Probably like this:
SELECT dbo.Table.FirstCol, dbo.Table.LastCol FROM Table
You get minor efficiency enhancements from specifying the table name and schema.
First: MIN() and MAX() on a text column gives AAAA and TTTT results which are not the first and last entries in my test table. They are the minimum and maximum values as mentioned.
I tried this (with .stats on) on my table which has over 94 million records:
select * from
(select col1 from mitable limit 1)
union
select * from
(select col1 from mitable limit 1 offset
(select count(0) from mitable) -1);
But it uses up a lot of virtual machine steps (281,624,718).
Then this which is much more straightforward (which works if the table was created without WITHOUT ROWID) [sql keywords are in capitals]:
SELECT col1 FROM mitable
WHERE ROWID = (SELECT MIN(ROWID) FROM mitable)
OR ROWID = (SELECT MAX(ROWID) FROM mitable);
That ran with 55 virtual machine steps on the same table and produced the same answer.
min()/max() approach is wrong. It is only correct, if the values are ascending only. I needed something liket this for currency rates, which are random raising and falling.
This is my solution:
select st.*
from stats_ticker st,
(
select min(rowid) as first, max(rowid) as last --here is magic part 1
from stats_ticker
-- next line is just a filter I need in my case.
-- if you want first/last of the whole table leave it out.
where timeutc between datetime('now', '-1 days') and datetime('now')
) firstlast
WHERE
st.rowid = firstlast.first --and these two rows do magic part 2
OR st.rowid = firstlast.last
ORDER BY st.rowid;
magic part 1: the subselect results in a single row with the columns first,last containing rowid's.
magic part 2 easy to filter on those two rowid's.
This is the best solution I've come up so far. Hope you like it.
We can do that by the help of Sql Aggregate function, like Max and Min. These are the two aggregate function which help you to get last and first element from data table .
Select max (column_name ), min(column name) from table name
Max will give you the max value means last value and min will give you the min value means it will give you the First value, from the specific table.