I'm trying to create a list of students whose behaviour is statistically worst across each of our school's year groups.
We have a table named students.
We then have behavioural flags and alerts, plus sanctions.
However, different categories of flag/alert/sanction are deemed more serious than others. These are stored with labels in their respective _categories table, e.g. flag_categories and sanction_categories. The flag table will then have a column called Category_ID (alerts is a bit different as it's just a Type field with 'A', 'C', 'P' and 'S' values).
If I want to look at data which shows our highest-flagged students in a specific year group, I'd run this query:
SELECT
CONCAT(stu.Firstname, " ", stu.Surname) AS `Student`,
COUNT(f.ID) AS `Flags`
FROM `students` stu
LEFT JOIN `flags` f ON f.Student_ID = stu.id
WHERE stu.Year_Group = 9
GROUP BY stu.id
ORDER BY `Flags` DESC
LIMIT 0, 20
If I wanted to show our students with the most Crisis alerts in a specific year group, I'd run this query:
SELECT
CONCAT(stu.Firstname, " ", stu.Surname) AS `Student`,
COUNT(f.ID) AS `Flags`
FROM `students` stu
LEFT JOIN `flags` f ON f.Student_ID = stu.id
WHERE stu.Year_Group = 9
AND f.Category_ID = 10
GROUP BY stu.id
ORDER BY `Flags` DESC
LIMIT 0, 20
If I want to find how many Late or Mobile flags a student has, and perhaps add these together (with weightings), I can run the following query:
SELECT
CONCAT(stu.Firstname, " ", stu.Surname) AS `Student`,
SUM(CASE WHEN f.Category_ID = 10 THEN 1 ELSE 0 END) AS `Late Flags`,
SUM(CASE WHEN f.Category_ID = 12 THEN 2 ELSE 0 END) AS `Mobile Flags`,
## not sure about this line below... is there a nicer way of doing it? `Late Flags` isn't recognised as a field apparently
## so I can't just do ( `Late Flags` + `Mobile Flags` )
(SUM(CASE WHEN f.Category_ID = 10 THEN 1 ELSE 0 END) + SUM(CASE WHEN f.Category_ID = 12 THEN 2 ELSE 0 END)) AS `Points`
FROM `flags` f
LEFT JOIN `students` stu ON f.Student_ID = stu.id
WHERE stu.Year_Group = 9
GROUP BY stu.id
ORDER BY `Points` DESC
LIMIT 0, 20
What I don't understand is how I would do this across myriad tables. I need to be able to weight:
Late (flags, Category_ID = 10), Absconded (flags, Category_ID = 15) and Community flags (flags, Category_ID = 13) plus Safeguarding alerts (alerts, Type = 'S') are all worth 1 point
Behavioural flags (flags, Category_ID IN (1, 7, 8)) are worth 2 points
Process alerts (alerts, Type = 'P') and detention sanctions (sanctions, Category_ID = 1) are worth 3 points
So on and so forth. That's far from an exhaustive list but I've included enough variables to help me get my head round a multi-table weighted sum.
The outcome I'm looking for is just 2 columns - student's name and weighted points.
So, according to the bullet points above, if a student has received 2 Late flags (1 point each) and 1 Process alert (3 points), the output should just say Joe Bloggs and 5.
Can anyone help me to understand how I can get these weighted values from different tables into one SUM'd output for each student?
[edit] SQLFiddle here: http://sqlfiddle.com/#!9/449218/1/0
Note, I am not doing this for the bounty. Please give to someone else.
This could be done with a few LEFT JOINs of derived tables. Note you did not supply the sanctions table. But the below would appear to be well illustrative. So I created a temp table. It would seem to allow for maximum flexibility without overcomplicating a larger left join notion that might be hard to debug. Afterall, you said your real querying will be much more complicated than this. As such, build out the temp table structure more.
This loads a tmp table up with default 0's for the students in the "passed by parameter Student Year" to a stored procedure. Two updates are performed. Then selects for a result set.
Schema / Load:
create schema s38741386; -- create a test database
use s38741386;
CREATE TABLE `students` (
`id` int(11) PRIMARY KEY,
`Firstname` varchar(50) NOT NULL,
`Surname` varchar(50) NOT NULL,
`Year_Group` int(2) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
# STUDENT INSERTS
INSERT INTO `students`
(`id`, `Firstname`, `Surname`, `Year_Group`)
VALUES
(201, 'Student', 'A', 9),
(202, 'Student', 'B', 9),
(203, 'Student', 'C', 9),
(204, 'Student', 'D', 9),
(205, 'Student', 'E', 9);
CREATE TABLE `alert` (
`ID` int(11) PRIMARY KEY,
`Staff_ID` int(6) NOT NULL,
`Datetime_Raised` datetime NOT NULL,
`Room_Label` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL,
`Type` enum('A','C','P','Q','S') COLLATE utf8_unicode_ci NOT NULL COMMENT 'A=Absconded, C=Crisis, P=Process, Q=Quiet, S=Safeguarding',
`Details` text COLLATE utf8_unicode_ci,
`Responder` int(8) DEFAULT NULL,
`Datetime_Responded` datetime DEFAULT NULL,
`Room_ID` int(11) NOT NULL COMMENT 'will be linked to internal room id.',
`Status` varchar(1) COLLATE utf8_unicode_ci DEFAULT NULL COMMENT 'O:ngoing, R:esolved'
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
# ALERT INSERTS
INSERT INTO `alert`
(`ID`, `Staff_ID`, `Datetime_Raised`, `Room_Label`, `Type`, `Details`, `Responder`, `Datetime_Responded`, `Room_ID`, `Status`)
VALUES
(1, '101', '2016-08-04 00:00:00', NULL, 'P', NULL, '103', '2016-08-04 00:00:01', '15', 'R'),
(2, '102', '2016-08-04 00:00:00', NULL, 'P', NULL, '103', '2016-08-04 00:00:01', '15', 'R'),
(3, '102', '2016-08-04 00:00:00', NULL, 'P', NULL, '103', '2016-08-04 00:00:01', '15', 'R'),
(4, '101', '2016-08-04 00:00:00', NULL, 'P', NULL, '103', '2016-08-04 00:00:01', '15', 'R');
CREATE TABLE `alert_students` (
`ID` int(11) PRIMARY KEY,
`Alert_ID` int(6) NOT NULL,
`Student_ID` int(6) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
# ALERT_STUDENT INSERTS
INSERT INTO `alert_students`
(`ID`, `Alert_ID`, `Student_ID`)
VALUES
(1, '1', '201'),
(2, '1', '202'),
(3, '2', '201'),
(4, '3', '202'),
(5, '4', '203'),
(6, '5', '204');
CREATE TABLE `flags` (
`ID` int(11) PRIMARY KEY,
`Staff_ID` int(11) NOT NULL,
`Student_ID` int(11) NOT NULL,
`Datetime` datetime NOT NULL,
`Category_ID` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
# ALERT INSERTS
-- TRUNCATE TABLE flags;
INSERT INTO `flags`
(`ID`, `Staff_ID`, `Student_ID`, `Datetime`, `Category_ID`)
VALUES
(1, '101', '201', '2016-08-04 00:00:01', 10),
(2, '102', '202', '2016-08-04 00:00:02', 12),
(3, '102', '203', '2016-08-04 00:00:03', 10),
(4, '101', '204', '2016-08-04 00:00:04', 13),
(5, '102', '202', '2016-08-04 00:00:02', 12),
(6, '102', '203', '2016-08-04 00:00:03', 10),
(7, '101', '204', '2016-08-04 00:00:04', 13),
(8, '102', '202', '2016-08-04 00:00:02', 10),
(9, '102', '203', '2016-08-04 00:00:03', 10),
(10, '101', '204', '2016-08-04 00:00:04', 7),
(11, '101', '204', '2016-08-04 00:00:07', 8),
(12, '101', '204', '2016-08-04 00:00:08', 1),
(13, '101', '204', '2016-08-04 00:00:09', 8);
Stored Procedure:
DROP PROCEDURE IF EXISTS rptSM_by_year;
DELIMITER $$
CREATE PROCEDURE rptSM_by_year
( pSY INT -- parameter student year
)
BEGIN
DROP TEMPORARY TABLE IF EXISTS tmpStudentMetrics;
CREATE TEMPORARY TABLE tmpStudentMetrics
( `StudentId` int(11) PRIMARY KEY,
LateFP INT NOT NULL,
MobiFP INT NOT NULL,
AbscFP INT NOT NULL,
CommFP INT NOT NULL,
SafeAP INT NOT NULL,
BehaFP INT NOT NULL,
ProcAP INT NOT NULL
)ENGINE=InnoDB;
INSERT tmpStudentMetrics (StudentId,LateFP,MobiFP,AbscFP,CommFP,SafeAP,BehaFP,ProcAP)
SELECT id,0,0,0,0,0,0,0
FROM students
WHERE Year_Group = pSY;
UPDATE tmpStudentMetrics tmp
JOIN
( SELECT
stu.id,
SUM(CASE WHEN f.Category_ID = 10 THEN 1 ELSE 0 END) AS `LateFP`,
SUM(CASE WHEN f.Category_ID = 15 THEN 1 ELSE 0 END) AS `AbscFP`,
SUM(CASE WHEN f.Category_ID = 13 THEN 1 ELSE 0 END) AS `CommFP`,
SUM(CASE WHEN f.Category_ID = 12 THEN 2 ELSE 0 END) AS `MobiFP`,
SUM(CASE WHEN f.Category_ID IN (1,7,8) THEN 2 ELSE 0 END) AS `BehaFP`
FROM `flags` f
LEFT JOIN `students` stu ON f.Student_ID = stu.id
WHERE stu.Year_Group = pSY
GROUP BY stu.id
) xDerived
ON xDerived.id=tmp.StudentId
SET tmp.LateFP=xDerived.LateFP,
tmp.AbscFP=xDerived.AbscFP,
tmp.CommFP=xDerived.CommFP,
tmp.MobiFP=xDerived.MobiFP,
tmp.BehaFP=xDerived.BehaFP;
UPDATE tmpStudentMetrics tmp
JOIN
( SELECT
stu.id,
SUM(CASE WHEN a.Type = 'S' THEN 1 ELSE 0 END) AS `SafeAP`,
SUM(CASE WHEN a.Type = 'P' THEN 3 ELSE 0 END) AS `ProcAP`
FROM `alert_students` als
JOIN `alert` a
ON a.ID=als.Alert_ID
JOIN `students` stu
ON stu.id=als.Student_ID and stu.Year_Group = pSY
GROUP BY stu.id
) xDerived
ON xDerived.id=tmp.StudentId
SET tmp.SafeAP=xDerived.SafeAP,
tmp.ProcAP=xDerived.ProcAP;
-- SELECT * FROM tmpStudentMetrics; -- check detail
SELECT stu.id,
CONCAT(stu.Firstname, " ", stu.Surname) AS `Student`,
tmp.LateFP+tmp.MobiFP+tmp.AbscFP+tmp.CommFP+tmp.SafeAP+tmp.BehaFP+tmp.ProcAP AS `Points`
FROM `students` stu
JOIN tmpStudentMetrics tmp
ON tmp.StudentId=stu.id
WHERE stu.`Year_Group` = pSY
ORDER BY stu.id;
-- SELECT * FROM tmpStudentMetrics; -- check detail
DROP TEMPORARY TABLE IF EXISTS tmpStudentMetrics;
-- TEMP TABLES are connection based. Explicityly dropped above for safety when done.
-- Depends on your connection type and life-span otherwise.
END$$
DELIMITER ;
Test:
call rptSM_by_year(9);
+-----+-----------+--------+
| id | Student | Points |
+-----+-----------+--------+
| 201 | Student A | 7 |
| 202 | Student B | 11 |
| 203 | Student C | 6 |
| 204 | Student D | 10 |
| 205 | Student E | 0 |
+-----+-----------+--------+
Cleanup:
drop schema s38741386; -- drop the test database
Think all you have asked can be done with a subquery and a couple of sub-SELECTs:
SELECT `Student`,
`Late Flags` * 1
+ `Absconded Flags` * 1
+ `Community Flags` * 1
+ `Safeguarding Alerts Flags` * 1
+ `Behavioural flags` * 2
+ `Process Alerts Flags` * 3 AS `Total Points`
FROM
(
SELECT
CONCAT(stu.Firstname, " ", stu.Surname) AS `Student`,
SUM(CASE WHEN f.Category_ID = 10 THEN 1 ELSE 0 END) AS `Late Flags`,
SUM(CASE WHEN f.Category_ID = 12 THEN 1 ELSE 0 END) AS `Mobile Flags`,
SUM(CASE WHEN f.Category_ID = 15 THEN 1 ELSE 0 END) AS `Absconded Flags`,
SUM(CASE WHEN f.Category_ID = 13 THEN 1 ELSE 0 END) AS `Community Flags`,
(SELECT COUNT(*) FROM `alert` a JOIN `alert_students` ast ON ast.`Alert_ID` = a.`ID`
WHERE ast.`Student_ID` = stu.`id` AND a.`Type` = 'S') AS `Safeguarding Alerts Flags`,
SUM(CASE WHEN f.Category_ID IN (1, 7, 8) THEN 1 ELSE 0 END) AS `Behavioural flags`,
(SELECT COUNT(*) FROM `alert` a JOIN `alert_students` ast ON ast.`Alert_ID` = a.`ID`
WHERE ast.`Student_ID` = stu.`id` AND a.`Type` = 'P') AS `Process Alerts Flags`
FROM `students` stu
LEFT JOIN `flags` f ON f.Student_ID = stu.id
WHERE stu.Year_Group = 9
GROUP BY stu.id
LIMIT 0, 20
) subq
ORDER BY `Total Points` DESC;
The above query includes everything you mentioned apart from sanctions (as your original SQL Fiddle demo didn't include this table).
Demo
An updated fiddle with the above query is here: http://sqlfiddle.com/#!9/449218/39.
You could use union all
Basically you create all your individual queries for each table and connect them all together using union all.
Here is an example, I used your student table twice but you would change the second one to what ever other table you want. SQLFiddle
You can do it with LEFT JOINS:
SELECT CONCAT(stu.firstname,' ', stu.surname) student,
COALESCE(f_group.weight_sum,0) + COALESCE(a_group.weight_sum,0) + COALESCE(s_group.weight_sum,0) points
FROM students stu
LEFT JOIN (
SELECT s_f.id, SUM(f.category_id IN (10,13,15) + 2 * f.category_id IN (1,7,8)) weight_sum
FROM students s_f
JOIN flags f
ON f.student_id = s_f.id
AND f.category_id IN (1,7,8,10,13,15)
WHERE s_f.year_group = :year_group
GROUP BY s_f.id
) f_group
LEFT JOIN (
SELECT s_a.id, 3 * COUNT(*) weight_sum
FROM students s_a
JOIN alerts a
ON a.student_id = s_a.id
AND a.type = 'P'
WHERE s_a.year_group = :year_group
GROUP BY s_a.id
) a_group
LEFT JOIN (
SELECT s_s.id, COUNT(*) weight_sum
FROM students s_s
JOIN sanctions s
ON s.student_id = s_s.id
AND s.category_id = 1
WHERE s_s.year_group = :year_group
GROUP BY s_s.id
) s_group
WHERE stu.year_group = :year_group
ORDER BY points DESC
LIMIT 0, 20
BUT if you have full access to the DB I'd be putting those weights in the respective categories and types, which will simplify the logic.
This is my table:
placeID | name | time
My goal is to remove the oldest records if the placeID has more than 3 record in the table.
It's pretty hard to understand, so example will make it clear:
1 | "Some Name" | *fresher timestamp*
1 | "Some Name" | *fresher timestamp*
1 | "Some Name" | *older timestamp* -- > Delete this result since there are more than 3 records, and this is the older than the rest
1 | "Some Name" | *older timestamp* -- > Delete this result since there are more than 3 records, and this is older than the rest
1 | "Some Name" | *fresher timestamp*
2 | "Some Name" | *fresher timestamp*
2 | "Some Name" | *fresher timestamp*
My trouble: How do I loop for each placeID that has more than 3 record in my table?
Removing the oldest results will not be a problem.
What I tried:
SELECT placeID,COUNT(placeID) FROM place_fbStatus WHERE count(placeID) > 3
Error: #1111 - Invalid use of group function
Another Try:
SELECT id, placeID
FROM place_fbStatus
HAVING COUNT( placeID ) >4
LIMIT 0 , 30
Problem: Show only one placeID
Here my Answer:
First thing is to create a PRIMARY KEY on your Table. So it is easy to DELETE a ROW with this.
Create the Table:
CREATE TABLE `mytable` (
`id` INT(11) UNSIGNED NOT NULL AUTO_INCREMENT,
`placeID` INT(11) DEFAULT NULL,
`name` VARCHAR(32) DEFAULT NULL,
`time` TIMESTAMP NULL DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `placeID` (`placeID`,`time`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;
Insert some stuff:
INSERT INTO `mytable` (`id`, `placeID`, `name`, `time`)
VALUES
(1, 1, 'Some Name', '2015-01-01 00:00:00'),
(2, 1, 'Some Name', '2015-01-02 00:00:00'),
(3, 1, 'Some Name', '2015-01-10 00:00:00'),
(4, 1, 'Some Name', '2015-01-09 00:00:00'),
(5, 2, 'Some Name', '2015-02-01 00:00:00'),
(6, 2, 'Some Name', '2015-01-30 00:00:00'),
(7, 1, 'Some Name', '2015-01-08 00:00:00');
i add rid ( ROW ID per Place ) and the Rows are in the right order
SELECT #rid:=IF( #pid = m.placeID, #rid+1 , 1) rid, m.*, #pid:=m.placeID AS pid
FROM mytable m, (SELECT #rid:=0, #pid:=-1) AS tmp
ORDER BY placeID,TIME ASC
And at last the Query to DELETE the ROWs:
DELETE mt FROM mytable mt
INNER JOIN (
SELECT * FROM (
SELECT #rid:=IF( #pid = m.placeID, #rid+1 , 1) rid, m.*, #pid:=m.placeID AS pid
FROM mytable m, (SELECT #rid:=0, #pid:=-1) AS tmp
ORDER BY placeID,TIME ASC
) AS x
WHERE x.rid >3
) AS dt ON mt.id = dt.id ;
You almost nailed it. What you need is to combine GROUP BY and HAVING
SELECT placeID, count(placeID) AS cnt
FROM place_fbStatus
GROUP BY placeID HAVING cnt>3
DELETE FROM Table WHERE ID NOT IN (SELECT TOP 3 ID FROM Table)
Refer to this thread
Although, it seems after trying it myself, it seems some versions of mysql dont yet support limit clauses in subqueries (This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'). To workaround this, try using an alternate subquery that doesnt involve limits.
Assuming a main "job" table, and two corresponding "log" tables (one for server events and the other for user events, with quite different data stored in each).
What would be the best way to return a selection of "job" records and the latest corresponding log record (with multiple fields) from each of the two "log" tables (if there are any).
Did get some inspiration from: MySQL Order before Group by
The following SQL would create some example tables/data...
CREATE TABLE job (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` tinytext NOT NULL,
PRIMARY KEY (id)
);
CREATE TABLE job_log_server (
`id` int(11) NOT NULL AUTO_INCREMENT,
`job_id` int(11) NOT NULL,
`event` tinytext NOT NULL,
`ip` tinytext NOT NULL,
`created` datetime NOT NULL,
PRIMARY KEY (id),
KEY job_id (job_id)
);
CREATE TABLE job_log_user (
`id` int(11) NOT NULL AUTO_INCREMENT,
`job_id` int(11) NOT NULL,
`event` tinytext NOT NULL,
`user_id` int(11) NOT NULL,
`created` datetime NOT NULL,
PRIMARY KEY (id),
KEY job_id (job_id)
);
INSERT INTO job VALUES (1, 'Job A');
INSERT INTO job VALUES (2, 'Job B');
INSERT INTO job VALUES (3, 'Job C');
INSERT INTO job VALUES (4, 'Job D');
INSERT INTO job_log_server VALUES (1, 2, 'Job B Event 1', '127.0.0.1', '2000-01-01 00:00:01');
INSERT INTO job_log_server VALUES (2, 2, 'Job B Event 2', '127.0.0.1', '2000-01-01 00:00:02');
INSERT INTO job_log_server VALUES (3, 2, 'Job B Event 3*', '127.0.0.1', '2000-01-01 00:00:03');
INSERT INTO job_log_server VALUES (4, 3, 'Job C Event 1*', '127.0.0.1', '2000-01-01 00:00:04');
INSERT INTO job_log_user VALUES (1, 1, 'Job A Event 1', 5, '2000-01-01 00:00:01');
INSERT INTO job_log_user VALUES (2, 1, 'Job A Event 2*', 5, '2000-01-01 00:00:02');
INSERT INTO job_log_user VALUES (3, 2, 'Job B Event 1*', 5, '2000-01-01 00:00:03');
INSERT INTO job_log_user VALUES (4, 4, 'Job D Event 1', 5, '2000-01-01 00:00:04');
INSERT INTO job_log_user VALUES (5, 4, 'Job D Event 2', 5, '2000-01-01 00:00:05');
INSERT INTO job_log_user VALUES (6, 4, 'Job D Event 3*', 5, '2000-01-01 00:00:06');
One option (only returning 1 field from each table) would be to use nested sub-queries... but the ORDER BY will have to be done in separate queries to the GROUP BY (x2):
SELECT
*
FROM
(
SELECT
s2.*,
jlu.event AS user_event
FROM
(
SELECT
*
FROM
(
SELECT
j.id,
j.name,
jls.event AS server_event
FROM
job AS j
LEFT JOIN
job_log_server AS jls ON jls.job_id = j.id
ORDER BY
jls.created DESC
) AS s1
GROUP BY
s1.id
) AS s2
LEFT JOIN
job_log_user AS jlu ON jlu.job_id = s2.id
ORDER BY
jlu.created DESC
) AS s3
GROUP BY
s3.id;
Which actually seems to perform quite well... just not very easy to understand.
Or you could try to return and sort the log records in two separate sub-queries:
SELECT
j.id,
j.name,
jls2.event AS server_event,
jlu2.event AS user_event
FROM
job AS j
LEFT JOIN
(
SELECT
jls.job_id,
jls.event
FROM
job_log_server AS jls
ORDER BY
jls.created DESC
) AS jls2 ON jls2.job_id = j.id
LEFT JOIN
(
SELECT
jlu.job_id,
jlu.event
FROM
job_log_user AS jlu
ORDER BY
jlu.created DESC
) AS jlu2 ON jlu2.job_id = j.id
GROUP BY
j.id;
But this seems to take quite a bit longer to run... possibly because of the amount of records it's adding to a temporary table, which are then mostly ignored (to keep this short-ish, I've not added any conditions to the job table, which would otherwise be only returning active jobs).
Not sure if I've missed anything obvious.
How about the following SQL Fiddle. It produces the same results as both of your queries.
SELECT j.id, j.name,
(
SELECT s.event
FROM job_log_server s
WHERE j.id = s.job_id
ORDER BY s.id DESC
LIMIT 1
)AS SERVER_EVENT,
(
SELECT u.event
FROM job_log_user u
WHERE j.id = u.job_id
ORDER BY u.id DESC
LIMIT 1
)AS USER_EVENT
FROM job j
EDIT SQL Fiddle:
SELECT m.id, m.name, js.event AS SERVER_EVENT, ju.event AS USER_EVENT
FROM
(
SELECT j.id, j.name,
(
SELECT s.id
FROM job_log_server s
WHERE j.id = s.job_id
ORDER BY s.id DESC
LIMIT 1
)AS S_E,
(
SELECT u.id
FROM job_log_user u
WHERE j.id = u.job_id
ORDER BY u.id DESC
LIMIT 1
)AS U_E
FROM job j
) m
LEFT JOIN job_log_server js ON js.id = m.S_E
LEFT JOIN job_log_user ju ON ju.id = m.U_E
I've data like this:
table 1
|id|fieldname1|fieldname2|price|
table 2
|id|fieldname3|fieldname4|price|
desired result:
|table1_id|table2_id|fieldname1|fieldname2|fieldname3|fieldname4|table1_min_price|table2_min_price|
I basically can do a left join from table 1 to table 2 on (fieldname1=fieldname3) getting the lowest price from each table.
However if table 2 return 0 queries there will not be joining no results at at all.
My intention is if table 2 or table 1 does not have results, it will still join up the table and create the fields with null values..
Any ideas how can this be done?
You may want a FULL OUTER JOIN which will give you either the values from Table1 or the Values from Table2 or Both. But I don't think you can get a row when its Neither
CHANGE 'UNION ALL' TO 'UNION' :
SELECT * from
(SELECT origin,destination,min(price) as oneway_cheapest FROM farestable_test where flight_type=0 group by origin,destination )as t1
LEFT JOIN
(SELECT origin,destination,min(price) as return_cheapest FROM farestable_test where flight_type=1 group by origin,destination )as t2
ON (t1.origin = t2.origin AND t1.destination=t2.destination)
UNION
SELECT * from
(SELECT origin,destination,min(price) as oneway_cheapest FROM farestable_test where flight_type=0 group by origin,destination )as t1
RIGHT JOIN
(SELECT origin,destination,min(price) as return_cheapest FROM farestable_test where flight_type=1 group by origin,destination )as t2
ON (t1.origin = t2.origin AND t1.destination=t2.destination)
Below is the table i've come up with.
CREATE TABLE IF NOT EXISTS `farestable_test` (
`id` tinyint(4) NOT NULL,
`origin` varchar(3) NOT NULL,
`destination` varchar(3) NOT NULL,
`flight_type` tinyint(4) NOT NULL,
`price` float NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
SELECT * from
(SELECT origin,destination,min(price) as oneway_cheapest FROM farestable_test where flight_type=0 group by origin,destination )as t1
LEFT JOIN
(SELECT origin,destination,min(price) as return_cheapest FROM farestable_test where flight_type=1 group by origin,destination )as t2
ON (t1.origin = t2.origin AND t1.destination=t2.destination)
UNION ALL
SELECT * from
(SELECT origin,destination,min(price) as oneway_cheapest FROM farestable_test where flight_type=0 group by origin,destination )as t1
RIGHT JOIN
(SELECT origin,destination,min(price) as return_cheapest FROM farestable_test where flight_type=1 group by origin,destination )as t2
ON (t1.origin = t2.origin AND t1.destination=t2.destination)
What i want to achieve is :
If data is
INSERT INTO `farestable_test` (`id`, `origin`, `destination`, `flight_type`, `price`) VALUES
(1, 'syd', 'mky', 0, 100),
(2, 'syd', 'mky', 0, 200),
It shows the CORRECT results:
origin destination oneway_cheapest origin destination return_cheapest
syd mky 100 null null null
AND THEN ....
If data is
INSERT INTO `farestable_test` (`id`, `origin`, `destination`, `flight_type`, `price`) VALUES
(3, 'syd', 'mky', 1, 300),
(4, 'syd', 'mky', 1, 400);
It wil show CORRECT result:
origin destination oneway_cheapest origin destination return_cheapest
null null null syd mky 300
However if data is below
INSERT INTO `farestable_test` (`id`, `origin`, `destination`, `flight_type`, `price`) VALUES
(1, 'syd', 'mky', 0, 100),
(2, 'syd', 'mky', 0, 200),
(3, 'syd', 'mky', 1, 300),
(4, 'syd', 'mky', 1, 400);
The result here is WRONG :
origin destination oneway_cheapest origin destination return_cheapest
syd mky 100 syd mky 300
syd mky 100 syd mky 300
It has duplicate data which is wrong, i cant do a full join without union all with mysql.
Is there a better way?
you can accomplish full outer join as below :
select *
from table1 left outer join table2 on(fieldname1=fieldname3)
union
select *
from table1 right outer join table2 on(fieldname1=fieldname3)