MySql Query for identifying sequence in a table - mysql

Question

I Need help with mysql query to update a new column of the same table based on series of entry and exit dates.
Below is table:
╔════╦═══════════╦═════════════╦═════════════╦════════════════════╗
║ ID ║ PLACE ║ ENTRYDATE ║ EXITDATE ║ LAST_PLACE_VISITED ║
╠════╬═══════════╬═════════════╬═════════════╬════════════════════╣
║ 1 ║ Delhi ║ 1-Jan-2012 ║ 5-Jan-2012 ║ ║
║ 1 ║ Agra ║ 10-Jan-2012 ║ 11-Jan-2012 ║ ║
║ 1 ║ Bangalore ║ 21-Jan-2012 ║ 24-Jan-2012 ║ ║
║ 1 ║ Mumbai ║ 12-Jan-2012 ║ 19-Jan-2012 ║ ║
║ 2 ║ LA ║ 1-Mar-2012 ║ 3-Mar-2012 ║ ║
║ 2 ║ SFO ║ 10-Mar-2012 ║ 14-Mar-2012 ║ ║
║ 2 ║ NY ║ 4-Mar-2012 ║ 9-Mar-2012 ║ ║
║ 3 ║ Delhi ║ 12-Apr-2012 ║ 13-Apr-2012 ║ ║
╚════╩═══════════╩═════════════╩═════════════╩════════════════════╝
The data type of ENTRYDATE and EXITDATE is DATE.
From the above table i need to write a query to update "Last_Place_Visited" column based on entry and exit date of the ID.
Any help with this query would be much appriciated.
Thanks.
Bhargav

Answer 1

Here's a very messy one since MySQL doesn't support window functions,
UPDATE TravelTbl a
INNER JOIN
(
SELECT a.ID,
a.Place,
a.EntryDate,
a.ExitDate,
b.Place Last_Place_Visited
FROM
(
SELECT ID,
Place,
EntryDate,
ExitDate,
Last_Place_Visited,
#grp := if(#ID = ID, #grp ,0) + 1 GRP_RecNo,
#ID := ID
FROM TravelTbl,
(SELECT #ID := '', #grp := 0) vars
ORDER BY EntryDate
) a
LEFT JOIN
(
SELECT ID,
Place,
EntryDate,
ExitDate,
Last_Place_Visited,
#grp2 := if(#ID2 = ID, #grp2 ,0) + 1 GRP_RecNo,
#ID2 := ID
FROM TravelTbl,
(SELECT #ID2 := '', #grp2 := 0) vars
ORDER BY EntryDate
) b ON a.ID = b.ID AND
a.GRP_RecNo = b.GRP_RecNo + 1
) b ON a.ID = b.ID AND
a.Place = b.Place AND
a.EntryDate = b.EntryDate AND
a.ExitDate = b.ExitDate AND
b.Last_Place_Visited IS NOT NULL
SET a.Last_Place_Visited = b.Last_Place_Visited
SQLFiddle Demo
OUTPUT
╔════╦═══════════╦═════════════╦═════════════╦════════════════════╗
║ ID ║ PLACE ║ ENTRYDATE ║ EXITDATE ║ LAST_PLACE_VISITED ║
╠════╬═══════════╬═════════════╬═════════════╬════════════════════╣
║ 1 ║ Delhi ║ 1-Jan-2012 ║ 5-Jan-2012 ║ (null) ║
║ 1 ║ Agra ║ 10-Jan-2012 ║ 11-Jan-2012 ║ Delhi ║
║ 1 ║ Bangalore ║ 21-Jan-2012 ║ 24-Jan-2012 ║ Mumbai ║
║ 1 ║ Mumbai ║ 12-Jan-2012 ║ 19-Jan-2012 ║ Agra ║
║ 2 ║ LA ║ 1-Mar-2012 ║ 3-Mar-2012 ║ (null) ║
║ 2 ║ SFO ║ 10-Mar-2012 ║ 14-Mar-2012 ║ NY ║
║ 2 ║ NY ║ 4-Mar-2012 ║ 9-Mar-2012 ║ LA ║
║ 3 ║ Delhi ║ 12-Apr-2012 ║ 13-Apr-2012 ║ (null) ║
╚════╩═══════════╩═════════════╩═════════════╩════════════════════╝

Answer 2

I tried to modify the table itself:
UPDATE T SET LAST_PLACE_VISITED = (
SELECT t2.PLACE
FROM T t2
WHERE t2.EXITDATE = (
SELECT MAX(t1.EXITDATE)
FROM T t1
WHERE t1.ID = ID
AND t1.EXITDATE < EXITDATE
));
MySQL won't permit this:
You can't specify target table 'T' for update in FROM clause:
But you could work with a view or a temporary table and use this:
UPDATE: Inserted LIMIT 1 for dealing with the case of multiple occurences of the maximum value of EXITDATE for distinct IDs. Disadvantage: We cannot predict which row with maximum value will be taken.
UPDATE 2: Added condition AND t2.ID = t0.ID
SELECT t0.ID, t0.PLACE, t0.ENTRYDATE, t0.EXITDATE, (
SELECT t2.PLACE
FROM T t2
WHERE t2.EXITDATE = (
SELECT MAX(t1.EXITDATE)
FROM T t1
WHERE t1.ID = t0.ID
AND t1.EXITDATE < t0.EXITDATE
)
AND t2.ID = t0.ID
LIMIT 1
) AS LAST_PLACE_VISITED
FROM T t0;
See my SQLFiddle demo