I need bit of a help in writing an SQL query.
A simple scenario is that I have a table named BookedRooms in which three columns are used most, checkInDate and checkOutDate, both are of type timestamp and roomId which is a foreign key to the Rooms table.
Now Rooms table has PK, name column and roomNo column.
This is BookedRooms table
+----+----------------------------+-------------------------+------------------+--+
| PK | checkInDate | checkOutDate | roomId | |
+----+----------------------------+-------------------------+------------------+--+
| 1 | 2022-05-26T00:00:00Z | 2022-05-29T00:00:00Z | 2 | |
| 2 | 2022-05-29T00:00:00Z | 2022-05-30T00:00:00Z | 3 | |
+----+----------------------------+-------------------------+------------------+--+
This is Rooms table
+----+------------+-------------------+--+
| PK | name | roomNo | |
+----+------------+-------------------+--+
| 2 | Deluxe | 102 | |
| 3 | King | 103 | |
+----+------------+-------------------+--+
Now, i wanna write a query in which if i put the month number like 4 , it tells me name and roomNo of Rooms which are free for each particular day of the month.
The logic to check if a room is occupied is that, if for example room 102 has a checkin date of 03 of month April and checkout date of 06 of month April , then the query will not include this room in the result set until the checkout date has come, only for that date and onwards would it include room 102 in the result set, again until this room appears in another checkInDate column somewhere.
Thank you
I recommend creating an exclusion constraint on bookedrooms. Not only can the GiST index that implements the constraint speed up the search you want, but it will also exclude double booking.
CREATE EXTENSION IF NOT EXISTS btree_gist;
ALTER TABLE bookedrooms ADD EXCLUDE USING gist (
tstzrange(checkindate, checkoutdate) WITH &&,
roomid WITH =
);
The query you need is
SELECT roomno FROM bookedrooms
EXCEPT
SELECT roomno FROM bookedrooms
WHERE tstzrange(checkindate, checkoutdate) &&
tstzrange(
date_trunc('year', current_timestamp) + INTERVAL '1 month' * 4,
date_trunc('year', current_timestamp) + INTERVAL '1 month' * (4 + 1)
);
&& is the "overlaps" operator for ranges.
Related
Lets say I have this table (balances) schema and data:
+----+---------+------------+
| id | balance | createdAt |
+----+---------+------------+
| 1 | 10 | 2021-11-18 |
| 2 | 12 | 2021-11-16 |
| 3 | 6 | 2021-11-04 |
+----+---------+------------+
To retrieve the last 7 days of balances, I would do something like this:
SELECT * FROM "balances" WHERE "createdAt" BETWEEN '2021-11-19T09:04:17.488Z' AND '2021-11-12T10:04:17.488Z' ORDER BY "createdAt" ASC
This will give me 2 records (IDs: 1 & 2), which is fine. However, what I'm looking at doing, probably with a second query, is to grab the record that is previous to that result set, by createdAt date, as my query is ordered by createdAt. Is there a way to do this with PG?
So whatever the time-range I use, I would also retrieve the record that is n-1 to the result set
To obtain the record you want, you may use a LIMIT query:
SELECT *
FROM balances
WHERE createdAt < '2021-11-19T09:04:17.488Z'
ORDER BY createdAt DESC
LIMIT 1;
This answer makes an assumption that there is only one record which is logically earlier than 2021-11-19T09:04:17.488Z, and there is no edge case of ties. If there are ties, we can break them by adding more levels to the ORDER BY clause.
I have a problem when I'm trying to reach the desired result. The task looks simple — make a daily count of occurrences of the event for top countries.
The main table looks like this:
id | date | country | col1 | col2 | ...
1 | 2018-01-01 21:21:21 | US | value 1 | value 2 | ...
2 | 2018-01-01 22:32:54 | UK | value 1 | value 2 | ...
From this table, I want to get daily event counts by the country, which is achieved by
SELECT date::DATE AT TIME ZONE 'UTC', country, COALESCE(count(id),0) FROM tab1
GROUP BY 1, 2
The problem comes when there is no event was made by an UK user on 2 January 2018
country_events
date | country | count
2018-01-01 | US | 23
2018-01-01 | UK | 5
2018-01-02 | US | 30
2018-01-02 | UK | 0 -> is desired result, but row is missing
I've tried to generate date series and series of countries which I'm looking for, then CROSS JOIN these two tables. This helper with columns date and country I've left joined with my result table like
SELECT * FROM helper h
LEFT JOIN country_events c ON c.date::DATE = h.date::DATE AND c.country = h.country
I'm using PostgreSQL.
You need an outer join, not a cross join:
SELECT tab1.date::date, tab1.country, coalesce(count(*), 0)
FROM generate_series(TIMESTAMP '2018-01-01 00:00:00',
TIMESTAMP '2018-01-31 00:00:00',
INTERVAL '1 day') AS ts(d)
LEFT JOIN tab1 ON tab1.date >= ts.d AND tab1.date < ts.d + INTERVAL '1 day'
GROUP BY tab1.date::date, tab1.country
ORDER BY tab1.date::date, tab1.country;
This will give the desired list for January 2018.
I have following table:
CREATE TABLE Kundendaten (
beschreiben_knr INTEGER REFERENCES Kunde(knr) DEFERRABLE INITIALLY DEFERRED,
erstelldatum DATE,
anschrift VARCHAR(40),
sonderrabat INTEGER,
PRIMARY KEY (erstelldatum, beschreiben_knr)
);
If i make this query:
select * from Kundendaten ORDER BY erstelldatum DESC;
i get:
beschreiben_knr | erstelldatum | anschrift | sonderrabat
-----------------+--------------+---------------+-------------
1 | 2015-11-01 | Winkelgasse 5 | 0
2 | 2015-11-01 | Badeteich 7 | 10
3 | 2015-11-01 | Senfgasse 7 | 15
1 | 2015-10-30 | Sonnenweg 3 | 5
But i need to get only the entry for the highest date entry if there are more then one. In this case the last row should not appear.
How can i achieve this in postgresql?
You want something like WHERE erstelldatum = MAX(DATE) but that doesn't work. You can use a sub-query to get the newest date.
SELECT *
FROM Kundendaten
WHERE erstelldatum = (
SELECT MAX(erstelldatum) FROM Kundendaten
);
(SQL Fiddle)
Postgres will optimize that subquery so it is only run once, but you'll want to make sure erstelldatum is indexed.
I have a table that contains data for every day in 2002, but it has some missing dates. Namely, 354 records for 2002 (instead of 365). For my calculations, I need to have the missing data in the table with Null values
+-----+------------+------------+
| ID | rainfall | date |
+-----+------------+------------+
| 100 | 110.2 | 2002-05-06 |
| 101 | 56.6 | 2002-05-07 |
| 102 | 65.6 | 2002-05-09 |
| 103 | 75.9 | 2002-05-10 |
+-----+------------+------------+
you see that 2002-05-08 is missing. I want my final table to be like:
+-----+------------+------------+
| ID | rainfall | date |
+-----+------------+------------+
| 100 | 110.2 | 2002-05-06 |
| 101 | 56.6 | 2002-05-07 |
| 102 | | 2002-05-08 |
| 103 | 65.6 | 2002-05-09 |
| 104 | 75.9 | 2002-05-10 |
+-----+------------+------------+
Is there a way to do that in PostgreSQL?
It doesn't matter if I have the result just as a query result (not necessarily an updated table)
date is a reserved word in standard SQL and the name of a data type in PostgreSQL. PostgreSQL allows it as identifier, but that doesn't make it a good idea. I use thedate as column name instead.
Don't rely on the absence of gaps in a surrogate ID. That's almost always a bad idea. Treat such an ID as unique number without meaning, even if it seems to carry certain other attributes most of the time.
In this particular case, as #Clodoaldo commented, thedate seems to be a perfect primary key and the column id is just cruft - which I removed:
CREATE TEMP TABLE tbl (thedate date PRIMARY KEY, rainfall numeric);
INSERT INTO tbl(thedate, rainfall) VALUES
('2002-05-06', 110.2)
, ('2002-05-07', 56.6)
, ('2002-05-09', 65.6)
, ('2002-05-10', 75.9);
Query
Full table by query:
SELECT x.thedate, t.rainfall -- rainfall automatically NULL for missing rows
FROM (
SELECT generate_series(min(thedate), max(thedate), '1d')::date AS thedate
FROM tbl
) x
LEFT JOIN tbl t USING (thedate)
ORDER BY x.thedate
Similar to what #a_horse_with_no_name posted, but simplified and ignoring the pruned id.
Fills in gaps between first and last date found in the table. If there can be leading / lagging gaps, extend accordingly. You can use date_trunc() like #Clodoaldo demonstrated - but his query suffers from syntax errors and can be simpler.
INSERT missing rows
The fastest and most readable way to do it is a NOT EXISTS anti-semi-join.
INSERT INTO tbl (thedate, rainfall)
SELECT x.thedate, NULL
FROM (
SELECT generate_series(min(thedate), max(thedate), '1d')::date AS thedate
FROM tbl
) x
WHERE NOT EXISTS (SELECT 1 FROM tbl t WHERE t.thedate = x.thedate)
Just do an outer join against a query that returns all dates in 2002:
with all_dates as (
select date '2002-01-01' + i as date_col
from generate_series(0, extract(doy from date '2002-12-31')::int - 1) as i
)
select row_number() over (order by ad.date_col) as id,
t.rainfall,
ad.date_col as date
from all_dates ad
left join your_table t on ad.date_col = t.date
order by ad.date_col;
This will not change your table, it will just produce the result as desired.
Note that the generated id column will not contain the same values as the ID column in your table as it is merely a counter in the result set.
You could also replace the row_number() function with extract(doy from ad.date_col)
To fill the gaps. This will not reorder the IDs:
insert into t (rainfall, "date") values
select null, "date"
from (
select d::date as "date"
from (
t
right join
generate_series(
(select date_trunc('year', min("date")) from t)::timestamp,
(select max("date") from t),
'1 day'
) s(d) on t."date" = s.d::date
where t."date" is null
) q
) s
You have to fully re-create your table as indexes haves to change.
The better way to do it is to use your prefered dbi language, make a loop ignoring ID and putting values in a new table with new serialized IDs.
for day in (whole needed calendar)
value = select rainfall from oldbrokentable where date = day
insert into newcleanedtable date=day, rainfall=value, id=serialized
(That's not real code! Just conceptual to be adapted to your prefered scripting language)
Related to - PostgreSQL DISTINCT ON with different ORDER BY
I have table purchases (product_id, purchased_at, address_id)
Sample data:
| id | product_id | purchased_at | address_id |
| 1 | 2 | 20 Mar 2012 21:01 | 1 |
| 2 | 2 | 20 Mar 2012 21:33 | 1 |
| 3 | 2 | 20 Mar 2012 21:39 | 2 |
| 4 | 2 | 20 Mar 2012 21:48 | 2 |
The result I expect is the most recent purchased product (full row) for each address_id and that result must be sorted in descendant order by the purchased_at field:
| id | product_id | purchased_at | address_id |
| 4 | 2 | 20 Mar 2012 21:48 | 2 |
| 2 | 2 | 20 Mar 2012 21:33 | 1 |
Using query:
SELECT DISTINCT ON (address_id) purchases.address_id, purchases.*
FROM "purchases"
WHERE "purchases"."product_id" = 2
ORDER BY purchases.address_id ASC, purchases.purchased_at DESC
I'm getting:
| id | product_id | purchased_at | address_id |
| 2 | 2 | 20 Mar 2012 21:33 | 1 |
| 4 | 2 | 20 Mar 2012 21:48 | 2 |
So the rows is same, but order is wrong. Any way to fix it?
Quite a clear question :)
SELECT t1.* FROM purchases t1
LEFT JOIN purchases t2
ON t1.address_id = t2.address_id AND t1.purchased_at < t2.purchased_at
WHERE t2.purchased_at IS NULL
ORDER BY t1.purchased_at DESC
And most likely a faster approach:
SELECT t1.* FROM purchases t1
JOIN (
SELECT address_id, max(purchased_at) max_purchased_at
FROM purchases
GROUP BY address_id
) t2
ON t1.address_id = t2.address_id AND t1.purchased_at = t2.max_purchased_at
ORDER BY t1.purchased_at DESC
Your ORDER BY is used by DISTINCT ON for picking which row for each distinct address_id to produce. If you then want to order the resulting records, make the DISTINCT ON a subselect and order its results:
SELECT * FROM
(
SELECT DISTINCT ON (address_id) purchases.address_id, purchases.*
FROM "purchases"
WHERE "purchases"."product_id" = 2
ORDER BY purchases.address_id ASC, purchases.purchased_at DESC
) distinct_addrs
order by distinct_addrs.purchased_at DESC
This query is trickier to rephrase properly than it looks.
The currently accepted, join-based answer doesn’t correctly handle the case where two candidate rows have the same given purchased_at value: it will return both rows.
You can get the right behaviour this way:
SELECT * FROM purchases AS given
WHERE product_id = 2
AND NOT EXISTS (
SELECT NULL FROM purchases AS other
WHERE given.address_id = other.address_id
AND (given.purchased_at < other.purchased_at OR given.id < other.id)
)
ORDER BY purchased_at DESC
Note how it has a fallback of comparing id values to disambiguate the case in which the purchased_at values match. This ensures that the condition can only ever be true for a single row among those that have the same address_id value.
The original query using DISTINCT ON handles this case automatically!
Also note the way that you are forced to encode the fact that you want “the latest for each address_id” twice, both in the given.purchased_at < other.purchased_at condition and the ORDER BY purchased_at DESC clause, and you have to make sure they match. I had to spend a few extra minutes to convince myself that this query is really positively correct.
It’s much easier to write this query correctly and understandbly by using DISTINCT ON together with an outer subquery, as suggested by dbenhur.
Try this !
SELECT DISTINCT ON (address_id) *
FROM purchases
WHERE product_id = 2
ORDER BY address_id, purchased_at DESC