I have a problem in finding the most and the least used value in the table.
The thing is that I have the values on two different columns and after I merge them together, I can't find the correct way of ordering them by the number of entries.
Firstly I have a table for each station
drop table if exists stations;
create table stations (
id_station int not null auto_increment,
st_name varchar (100),
primary key (id_station)
)auto_increment = 2000;
insert into stations values (null, 'Kobenhavn H'),
(null, 'Orestad'),
(null, 'Tarnby'),
(null, 'CPH Lufthavn');
select * from stations;
The table with the tickets contains all the information about the stations in use.
drop table if exists tickets;
create table tickets (
id_ticket int not null auto_increment,
starting_point varchar (100),
ending_point varchar (100),
id_train int,
departure_date date,
primary key (id_ticket),
foreign key (id_train) references trains (id_train)
)auto_increment = 100;
As you can see, the most used value is 2000 which is used 4 times; and the least used value is 2001 which is only used 1 time.
insert into tickets values ( null, 2000, 2003, 1, '2018.05.14'),
( null, 2000, 2003, 1, '2018.05.14'),
( null, 2003, 2000, 1, '2018.05.18'),
( null, 2002, 2000, 3, '2018.06.2 '),
( null, 2001, 2002, 3, '2018.06.4 ');
select * from tickets;
Here I must find the least and the most crowded station
I have tried to union all in order to get the two columns into one.
(select starting_point as 'All stations in use' from tickets)
union all
(select ending_point from tickets);
The part from above works perfectly, but the problem is that I don't know how to link it with the following part.
SELECT
stations.id_station AS 'All stations in use',
COUNT(*) AS 'Number of passengers'
FROM
(SELECT starting_point
FROM tickets
UNION ALL
SELECT ending_point from tickets) as Unix, stations
GROUP BY stations.id_station
ORDER BY COUNT(*);
What I wanted to obtain is:
2000 4
2003 3
2002 2
2001 1
Where 2000, 2003, 2002 and 2001 are the stations in use either from the first query or from the table stations
But what I get is:
2000 10
2001 10
2002 10
2003 10
I hoped that I could use something like where stations.id_station = Unix in the hope that I will solve the problem, but it won't work and I get the following error
Error Code: 1054. Unknown column 'Unix' in 'where clause'.
Can anyone help me please? I have been working on it for hours and I couldn't find any solution...
Kind regards,
Alin Chiver
You could use a union all
select point, count(*) from (
select starting_point point
from tickets
union all
select ending_point
from tickets) t
group by point
Related
I need help with an assignment for school. I've only been doing SQL for 2 months and can't figure this out. My teacher gave me some hints about using self joins. The database has 3 more tables, but I don't think they are needed here.
The assignment is to write a query that will help hotel staff find double bookings (same room, same date). I've made a test database that has a double booking to control the query.
drop database if exists hoteltest;
create database hoteltest;
use hoteltest;
create table Roomreservation(
ResNr int not null,
RoomNr int not null,
FromDate date not null,
ToDate date not null,
primary key (ResNr, RoomNr, FromDate)
);
insert into Roomreservation
values
(51, 102, '2008-12-05', '2008-12-07'),
(51, 103, '2008-12-05', '2008-12-07'),
(51, 104, '2008-12-05', '2008-12-09'),
(52, 201, '2008-12-05', '2008-12-14'),
(53, 102, '2008-12-04', '2008-12-10');
select * from Roomreservation;
Does anyone have a good and easy solution for this?
Honestly, I'm kinda stuck, I've been trying different solutions with concat_ws and the dates but with no results.
You need to check for the range of Fromdate and todate. For this, you can use between. You also exclude the same row from the join. And you only extract one from the join since AxB will lead to BxA on the join. Therefore :
SELECT t1.* FROM Roomreservation t1
JOIN Roomreservation t2 ON t1.ResNr<>t2.ResNr AND t1.RoomNr=t2.RoomNr AND (t1.FromDate BETWEEN t2.FromDate AND t2.ToDate OR t2.FromDate BETWEEN t1.FromDate AND t1.ToDate);
You should use an auto-incremented primary key, such as something as the resnumber but unique.
The idea is to join the table on itself, looking for 2 records where the ResNo is different, the RoomNr is the same, and either the FromDate or the ToDate is between the other record's FromDate-ToDate.
I could write the query for you, but i believe in self-learning, especially for school assignments (rather than actual work).
NOTE: don't get lazy when it comes to naming, e.g. ResNr -> ReservationNumber
will save you more time in the future when reading queries
There is a, or at least a, double booking on the following dates:
RoomNr
d
count(*)
102
2008-12-04
2
102
2008-12-05
2
102
2008-12-06
2
102
2008-12-07
2
102
2008-12-08
2
102
2008-12-09
2
102
2008-12-10
2
102
2008-12-11
2
102
2008-12-12
2
102
2008-12-13
2
102
2008-12-14
2
WITH recursive dates as (
select min(FromDate) as d FROM Roomreservation
union all
select adddate(d, INTERVAL 1 day)
from dates
where d<(select max(ToDate) from Roomreservation)
)
SELECT
RoomNr,
d,
count(*)
FROM Roomreservation
CROSS JOIN dates
GROUP BY
RoomNr,
d
HAVING count(*) >1
ORDER BY d, RoomNr
The WITH recursive.... is creating a table (dates) with all dates from the lower value of FromDate to the highest value of ToDate.
And then it's just a matter of counting how many reservation there are on that day, for that RoomNr.
see: DBFIDDLE
I have this MySQL table:
CREATE TABLE bills
(
id_interess INT UNSIGNED NOT NULL,
id_bill VARCHAR(30) NULL,
PRIMARY KEY (id_interess)
) ENGINE=InnoDB;
And now I want to be able to manually insert unique integer for id_interess and automatically generate id_bill so that it consists of a current date and an integer (integer resets on a new year using trigger) like this:
id_interess |id_bill |
------------+-----------+
1 |20170912-1 |
2 |20171030-2 |
6 |20171125-3 |
10 |20171231-4 |
200 |20180101-1 |
3 |20180101-2 |
8 |20180102-3 |
If anyone has direct solution to this using only one query, I would be very glad! I only came up with a solution that uses three queries, but I still get some errors...
My newbie attempt: I created an additional column id_bill_tmp which holds integer part of id_bill like this:
CREATE TABLE bill
(
id_interess INT UNSIGNED NOT NULL,
id_bill_tmp INT UNSIGNED NULL,
id_bill VARCHAR(30) NULL,
PRIMARY KEY (id_interess)
) ENGINE=InnoDB;
Table from above would in this case look like this (note that on new year id_bill_tmp is reset to 1 and therefore I can't use AUTO_INCREMENT which can only be used on keys and keys need unique values in a column):
id_interess |id_bill_tmp |id_bill |
------------+--------------+-----------+
1 |1 |20170912-1 |
2 |2 |20171030-2 |
6 |3 |20171125-3 |
10 |4 |20171231-4 |
200 |1 |20180101-1 |
3 |2 |20180101-2 |
6 |3 |20180102-3 |
So for example to insert 1st row from the above table, table would have to be empty, and I would insert a value in three queries like this:
1st query:
INSERT INTO racuni (id_interess) VALUES (1);
I do this first because I don't know how to increment a nonexistent value for id_bill_tmp and this helped me to first get id_bill_tmp = NULL:
id_interess |id_bill_tmp |id_bill |
------------+--------------+-----------+
1 |[NULL] |[NULL] |
2nd query
Now I try to increment id_bill_tmp to become 1 - I tried two queries both fail saying:
table is specified twice both as a target for 'update' and as a separate source for data
This are the queries I tried:
UPDATE bills
SET id_bill_tmp = (SELECT IFNULL(id_bill_tmp, 0)+1 AS id_bill_tmp FROM bills)
WHERE id_interess = 1;
UPDATE bills
SET id_bill_tmp = (SELECT max(id_bill_tmp)+1 FROM bills)
WHERE id_interess = 1;
3rd query:
The final step would be to reuse id_bill_tmp as integer part of id_bill like this:
UPDATE bills
SET id_bill = concat(curdate()+0,'-',id_bill_tmp)
WHERE id_interess = 1;
so that I finally get
id_interess |id_bill_tmp |id_bill |
------------+--------------+-----------+
1 |1 |20170912-1 |
So if anyone can help me with the 2nd query or even present a solution with a single query or even without using column id_bill_tmp it would be wonderful.
Solution #1 - with the extra column
Demo
http://rextester.com/GOTPA70741
SQL
INSERT INTO bills (id_interess, id_bill_tmp, id_bill) VALUES (
1, -- (Change this value appropriately for each insert)
IF(LEFT((SELECT id_bill FROM
(SELECT MAX(CONCAT(LEFT(id_bill, 8),
LPAD(SUBSTR(id_bill, 10), 10, 0))) AS id_bill
FROM bills) b1), 4) = DATE_FORMAT(CURDATE(),'%Y'),
IFNULL(
(SELECT id_bill_tmp
FROM (SELECT id_bill_tmp
FROM bills
WHERE CONCAT(LEFT(id_bill, 8),
LPAD(SUBSTR(id_bill, 10), 10, 0)) =
(SELECT MAX(CONCAT(LEFT(id_bill, 8),
LPAD(SUBSTR(id_bill, 10), 10, 0)))
FROM bills)) b2),
0),
0)
+ 1,
CONCAT(DATE_FORMAT(CURDATE(),'%Y%m%d'), '-' , id_bill_tmp));
Notes
The query looks slightly more complicated that it actually is because of the issue that MySQL won't let you directly use a subselect from the same table that's being inserted into. This is circumvented using the method of wrapping another subselect around it as described here.
Solution #2 - without the extra column
Demo
http://rextester.com/IYES40010
SQL
INSERT INTO bills (id_interess, id_bill) VALUES (
1, -- (Change this value appropriately for each insert)
CONCAT(DATE_FORMAT(CURDATE(),'%Y%m%d'),
'-' ,
IF(LEFT((SELECT id_bill
FROM (SELECT MAX(CONCAT(LEFT(id_bill, 8),
LPAD(SUBSTR(id_bill, 10), 10, 0))) AS id_bill
FROM bills) b1), 4) = DATE_FORMAT(CURDATE(),'%Y'),
IFNULL(
(SELECT id_bill_tmp
FROM (SELECT SUBSTR(MAX(CONCAT(LEFT(id_bill, 8),
LPAD(SUBSTR(id_bill, 10), 10, 0))), 9)
AS id_bill_tmp
FROM bills) b2),
0),
0)
+ 1));
Notes
This is along the same lines as above but gets the numeric value that would have been in id_bill_tmp by extracting from the right part of id_bill from the 10th character position onwards via SUBSTR(id_bill, 10).
Step by step breakdown
CONCAT(...) assembles the string by concatenating its parts together.
DATE_FORMAT(CURDATE(),'%Y%m%d') formats the current date as yyyymmdd (e.g. 20170923).
The IF(..., <x>, <y>) is used to check whether the most recent date that is already present is for the current year: If it is then the numeric part should continue by incrementing the sequence, otherwise it is reset to 1.
LEFT(<date>, 4) gets the year from the most recent date - by extracting from the first 4 characters of id_bill.
SELECT MAX(...) AS id_bill FROM bills gets the most recent date + sequence number from id_bill and gives this an alias of id_bill. (See the notes above about why the subquery also needs to be given an alias (b1) and then wrapped in another SELECT). See the two steps below for how a string is constructed such that MAX can be used for the ordering.
CONCAT(LEFT(id_bill, 8), ...) is constructing a string that can be used for the above ordering by combining the date part with the sequence number padded with zeros. E.g. 201709230000000001.
LPAD(SUBSTR(id_bill, 10), 10, 0) pads the sequence number with zeros (e.g. 0000000001 so that MAX can be used for the ordering. (See the comment by Paul Spiegel to understand why this needs to be done - e.g. so that sequence number 10 is ordered just after 9 rather than just after 1).
DATE_FORMAT(CURDATE(),'%Y') formats the current date as a year (e.g. 2017) for the IF comparison mentioned in (3) above.
IFNULL(<x>, <y>) is used for the very first row since no existing row will be found so the result will be NULL. In this case the numeric part should begin at 1.
SELECT SUBSTR(MAX(...), 9) AS id_bill_tmp FROM bills selects the most recent date + sequence number from id_bill (as described above) and then extracts its sequence number, which is always from character position 9 onwards. Again, this subquery needs to be aliased (b2) and wrapped in another SELECT.
+ 1 increments the sequence number. (Note that this is always done since 0 is used in the cases described above where the sequence number should be set to 1).
If you are certain to be inserting in chronological order, then this will both bump the number and eliminate the need for the annual trigger:
DROP FUNCTION fcn46309431;
DELIMITER //
CREATE FUNCTION fcn46309431 (_max VARCHAR(22))
RETURNS VARCHAR(22)
DETERMINISTIC
SQL SECURITY INVOKER
BEGIN
RETURN
CONCAT(DATE_FORMAT(CURDATE(), "%Y%m%d"), '-',
IF( LEFT(_max, 4) = YEAR(CURDATE()),
SUBSTRING_INDEX(_max, '-', -1) + 1,
1 ) );
END
//
DELIMITER ;
INSERT INTO se46309431 (id_interess, id_bill)
SELECT 149, fcn46309431(MAX(id_bill)) FROM se46309431;
SELECT * FROM se46309431;
(If you might insert out of date order, then the MAX(..) can mess up.)
A similar solution is shown here: https://www.percona.com/blog/2008/04/02/stored-function-to-generate-sequences/
What you could do is to create a sequence with table, as shown there:
delimiter //
create function seq(seq_name char (20)) returns int
begin
update seq set val=last_insert_id(val+1) where name=seq_name;
return last_insert_id();
end
//
delimiter ;
CREATE TABLE `seq` (
`name` varchar(20) NOT NULL,
`val` int(10) unsigned NOT NULL,
PRIMARY KEY (`name`)
)
Then you need to populate the sequence values for each year, like so:
insert into seq values('2017',1);
insert into seq values('2018',1);
insert into seq values('2019',1);
...
(only need to do this once)
Finally, this should work:
insert into bills (id_interess, id_bill)
select
123,
concat(date_format(now(), '%Y%m%d-'), seq(date_format(now(), '%Y')));
Just replace 123 with some real/unique/dynamic id and you should be good to go.
I think you should redesign your approach to make life easier.
I would design your table as follows:
id_interess |id_counter |id_bill |
------------+--------------+-----------+
1 |1 |20170912 |
2 |2 |20171231 |
3 |1 |20180101 |
Your desired output for the first row would be "20170912-1", but you would merge id_counter and id_bill in your SQL-Query or in your application logic, not directly in a table (here is why).
Now you can write your SQL-Statements for that table.
Furthermore, I would advise not to store the counter in the table. You should only read the records' id and date from your database and calculate the id_counter in your application (or even in your SQL-Query).
You could also declare your column id_counter as auto_increment and reset it each time, see here.
One approach to do in single query would be just save the date in your table when ever you update any record. For id_bill no., generate a sequence when you want to display the records.
Schema
CREATE TABLE IF NOT EXISTS `bill` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
`bill_date` date NULL
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
Query
select a.id,concat(DATE_FORMAT(a.bill_date,"%Y%m%d"),'-',a.no) id_bill
from(
select b.*,count(b2.bill_date) no
from bill b
join bill b2 ON (EXTRACT(YEAR FROM b.bill_date) = EXTRACT(YEAR FROM b2.bill_date)
and b.bill_date >= b2.bill_date)
group by b.id
order by b.bill_date,no
) a
Inner query will return you the rank of each record per year by joining the same table outer query just format the data as per your desired view
DEMO
If for same date there can be more than 1 entries then in inner query the id column which is set to auto_increment can be used to handle this case
Updated Query
select a.id,concat(DATE_FORMAT(a.bill_date,"%Y%m%d"),'-',a.no) id_bill
from(
select b.*,count(b2.bill_date) no
from bill b
join bill b2 ON (EXTRACT(YEAR FROM b.bill_date) = EXTRACT(YEAR FROM b2.bill_date)
and b.id >= b2.id)
group by b.id
order by b.bill_date,no
) a
Updated Demo
The following solution requires generated (virtual) columns (available in MySQL 5.7 and MariaDB).
CREATE TABLE bills (
id_interess INT UNSIGNED NOT NULL,
bill_dt DATETIME DEFAULT CURRENT_TIMESTAMP,
bill_year YEAR AS (year(bill_dt)),
year_position INT UNSIGNED NULL,
id_bill VARCHAR(30) AS (concat(date_format(bill_dt, '%Y%m%d-'), year_position)),
PRIMARY KEY (id_interess),
INDEX (bill_year, year_position)
) ENGINE=InnoDB;
bill_year and id_bill are not stored in the table. They are derived from other columns. However - bill_year is stored in the index, which we need to get the last position for a specific year efficiently (it would also work without the index).
To insert a new row with the current timestamp:
insert into bills(id_interess, year_position)
select 1, coalesce(max(year_position), 0) + 1
from bills
where bill_year = year(now());
You can also use a custom timestamp or date:
insert into bills(id_interess, bill_dt, year_position)
select 10, '2016-01-01', coalesce(max(year_position), 0) + 1
from bills
where bill_year = year('2016-01-01')
Demo: https://www.db-fiddle.com/f/8pFKQb93LqNPNaD5UhzVwu/0
To get even simpler inserts, you can create a trigger which will calculate year_postion:
CREATE TRIGGER bills_after_insert BEFORE INSERT ON bills FOR EACH ROW
SET new.year_position = (
SELECT coalesce(max(year_position), 0) + 1
FROM bills
WHERE bill_year = year(coalesce(new.bill_dt, now()))
);
Now your insert statement would look like:
insert into bills(id_interess) values (1);
or
insert into bills(id_interess, bill_dt) values (11, '2016-02-02');
And the select statements:
select id_interess, id_bill
from bills
order by id_bill;
Demo: https://www.db-fiddle.com/f/55yqMh4E1tVxbpt9HXnBaS/0
Update
If you really, really need to keep your schema, you can try the following insert statement:
insert into bills(id_interess, id_bill)
select
#id_interess,
concat(
date_format(#date, '%Y%m%d-'),
coalesce(max(substr(id_bill, 10) + 1), 1)
)
from bills
where id_bill like concat(year(#date), '%');
Replace #id_interess and #date accordingly. For #date you can use CURDATE() but also any other date you want. There is no issue inserting dates out of order. You can even insert dates from 2016 when entries for 2017 already exist.
Demo: http://rextester.com/BXK47791
The LIKE condition in the WHERE clause can use an index on id_bill (if you define it), so the query only need to read the entries from the same year. But there is no way to determine the last counter value efficiently with this schema. The engine will need to read all rows for the cpecified year, extract the counter and search for the MAX value. Beside the complexity of the insert statement, this is one more reason to change the schema.
I have a table which contains Bank ID (not unique) and Bank names in a table. If the bank name contains keyword 'bank' I want to rank it one and if it
has 'financial' rank it two and 'Trust' gets a rank of three.
Create table dbo.banknames(id int null,bankname varchar(200) null)
insert into dbo.bankname(1,'U.S. Trust')
insert into dbo.bankname(1,'Bank of america')
insert into dbo.bankname(1,'BOA Financial services')
insert into dbo.bankname(2,'Citizens trust')
insert into dbo.bankname(3,'People''s bank trust')
insert into dbo.bankname(3,'People''s financial service')
The output should look like
Create table dbo.ExpectedOUTPUTbanknames(id int null,bankname varchar(200) null,rank int null)
insert into dbo.ExpectedOUTPUTbankname(1,'U.S. Trust',3)
insert into dbo.ExpectedOUTPUTbankname(1,'Bank of america',1)
insert into dbo.ExpectedOUTPUTbankname(1,'BOA Financial services',2)
insert into dbo.ExpectedOUTPUTbankname(2,'Citizens trust',3)
insert into dbo.ExpectedOUTPUTbankname(3,'People''s bank trust',1)
insert into dbo.ExpectedOUTPUTbankname(3,'People''s financial service',2)
select *
into dbo.OUTPUTbankname
(
SELECT *,1 as RNK FROM dbo.banknames
WHERE (bankname LIKE '%bank%')
UNION
SELECT *,1 as RNK FROM dbo.banknames
WHERE (bankname LIKE '%financial%')
UNION
SELECT *,1 as RNK FROM dbo.banknames
WHERE (bankname LIKE '%trust%')
) qrey
For bankid =3, since bankname has both keywords bank and trust, I get 3 rows, 'People''s bank trust' with rank=1 'People''s financial service'with rank=2
'People''s bank trust' with rank=3. How can i avoid this
Thanks
MR
Use a CASE expression:
SELECT id,
bankname,
CASE WHEN bankname LIKE '%[Bb]ank%' THEN 1
WHEN bankname LIKE '%[Ff]inancial%' THEN 2
WHEN bankname LIKE '%[Tt]rust%' THEN 3
END AS rank
FROM dbo.banknames
Fortunately SQL Server supports character ranges in its LIKE expressions, making it easy to match the keywords regardless of whether the first letter be capitalized or not.
THE DATA
I have a set of data in the following format:
CAR_MAKE TABLE
ID MAKE
1 Ford
2 Tesla
3 Acura
4 Honda
5 Toyota
MAKE_NOTES TABLE
NOTE_ID MAKE_ID MAKE_NAME NOTE
1 1 Ford New QNX-Based Sync System
2 2 Tesla Looking forward to Model 3
3 5 Toyota Updated Corolla 2018
4 Null Ferrari Very Fast and Very Red
I know there's repeating data (make_name) between table 1 and table 2. Let's assume I can't mess with the data. I also can't guarantee that an entry would have been made in car_make first. In such a case make_notes.make_id should be null.
WHAT I'VE DONE SO FAR
What I'm trying to do is INSERT a row into make_notes, inserting null into make_id if it does not exist in car_make, otherwise inserting car_make.id.
This works fine if, make_name exists in car_make... but if I attempt to insert a record with a make_name that does not exist in car_make, no record is inserted (no error is thrown either).
INSERT INTO make_notes (
make_id,
make_name,
note
)
SELECT
id,
'ford',
'New Note'
FROM car_make
WHERE make = 'ford';
I've also tried to use this as a subquery:
SELECT
CASE
WHEN (SELECT EXISTS (SELECT 1 FROM car_make where make = 'Ferrari') = 1)
THEN car_make.id
ELSE null
END AS make_id
FROM car_make;
I have not been albe intergrate it into my main query without throwing an error. As a stand alone query it returns one row for each entry in car_make, as null if 'Ferrari' does not exist, and each id if it does.
QUESTION
How do I create an insert query that will insert into 'make_notes', and insert 'null' if make_name does not exist in car_make, and insert car_make.id if make_name does exist?
I think using an IF() clause should work for your case:
INSERT INTO `make_notes` (
make_id,
make_name,
note
)
VALUES(
IF(
((SELECT COUNT(*) FROM `car_make` WHERE `make` = 'ford') > 0),
(SELECT `id` FROM `car_make` WHERE `make` = 'ford'),
NULL
),
'ford',
'New note'
);
I don't think you can do that with one query. I think you have to run a select to get the ID from the car_make table. Then you do the insert.
The following table will store exchange rates between various currencies over time:
CREATE TABLE `currency_exchange` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`currency_from` int(11) DEFAULT NULL,
`currency_to` int(11) DEFAULT NULL,
`rate_in` decimal(12,4) DEFAULT NULL,
`rate_out` decimal(12,4) DEFAULT NULL,
`exchange_date` datetime DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
How would I query it to fetch a list the most recent exchange rates?
It'd essentially be identifying the combined currency_from and currency_to columns as a distinct exchange rate, then finding the one with the most recent date.
For example, let's say I've got the data as:
INSERT INTO currency_exchange (id, currency_from, currency_to, rate_in, rate_out, exchange_date) VALUES
(1, 1, 2, 0.234, 1.789, '2012-07-23 09:24:34'),
(2, 2, 1, 0.234, 1.789, '2012-07-24 09:24:34'),
(3, 2, 1, 0.234, 1.789, '2012-07-24 09:24:35'),
(4, 1, 3, 0.234, 1.789, '2012-07-24 09:24:34');
I'd want it to select row ID's:
1 - as the most recent rate between currencies 1 and 2
3 - as the most recent rate between currencies 2 and 1
4 - as the most recent rate between currencies 1 and 3
The following query should work:
SELECT ce.*
FROM currency_exhcnage ce
LEFT JOIN currency_exchange newer
ON (newer.currency_from = ce.currency_from
AND newer.currency_to = ce.currency_to
AND newer.exchange_date > ce.exchange_date)
WHERE newer.id IS NULL
The trick of doing a self-LEFT JOIN is to avoid resorting to a subquery that may be very expensive if you have large datasets. It's essentially looking for records where no "newer" record exists.
Alternatively, you could go for a simpler (although it may (or may not, as noted in comments) be slower):
SELECT *
FROM currency_exchange ce
NATURAL JOIN (
SELECT currency_from, currency_to, MAX(exchange_date) AS exchange_date
FROM currency_exchange
GROUP BY currency_from, currency_to
) AS most_recent
Insert the values of $currency_from and $currency_to in your dynamic query.
The query below will return the nearest row to the current time
SELECT id FROM currency_exchange WHERE currency_from='$currency_from' AND currency_to='$currency_to' ORDER BY ABS( DATEDIFF( exchange_date, now() ) ) LIMIT 1