find nearest row of different type in oracle - oracle

Question

My table looks like
__ Key type timeStamp flag
1 ) 1 B 2015-06-28 22:19:26 Y
2 ) 1 B 2015-06-28 22:20:22 Y
3 ) 1 C 2015-06-28 22:22:06 N
4 ) 1 A 2015-06-28 22:25:11 N
5 ) 1 B 2015-06-28 22:29:44 Y
6 ) 1 A 2015-06-28 22:33:33 N
7 ) 1 B 2015-06-28 22:35:21 N
8 ) 1 B 2015-06-28 22:39:34 Y
9 ) 1 B 2015-06-28 22:43:53 N
10) 1 A 2015-06-28 22:45:53 N
I need to find out all the types of A whose flag='N' with respect to which there exist type B whose timestampOF(B)<timestampOF(A) and Flag(B)='Y' and key(A)=key(B).
note: If there exist two B previous than A than take the B with max timestamp.(ROW[8,9,10] 9 is taken instead of 8)
OUTPUT
__ Key type timeStamp flag
4 ) 1 A 2015-06-28 22:25:11 N
6 ) 1 A 2015-06-28 22:33:33 N
My approach
SELECT *
FROM tab TAB_OUT
WHERE TAB_OUT.TYPE='A'
AND TAB_OUT.FLAG='N'
AND EXISTS(
SELECT *
FROM tab TAB_IN
WHERE TAB_IN.KEY = TAB_OUT.KEY
AND TAB_IN.TYPE='B'
AND TAB_OUT.FLAG='Y'
AND TAB_IN.timestamp<TAB_OUT.timestamp
AND TAB_IN.timestamp = (SELECT MAX(timestamp) from
tab where timestamp< `TAB_OUT.timestamp`)
);
But in this i can not use TAB_OUT.timestamp in third level query. Is there any alternative solution to solve this problem.
In my query note: part is not satisfied as my query as it skips no. 9) and satisfy condition with no. 8)

Answer 1

A solution that only requires a single table scan:
SQL Fiddle
Oracle 11g R2 Schema Setup:
CREATE TABLE table_name ( Key, type, timeStamp, flag ) AS
SELECT 1, 'B', CAST( TIMESTAMP '2015-06-28 22:19:26' AS DATE ), 'Y' FROM DUAL
UNION ALL SELECT 1, 'B', CAST( TIMESTAMP '2015-06-28 22:20:22' AS DATE ), 'Y' FROM DUAL
UNION ALL SELECT 1, 'C', CAST( TIMESTAMP '2015-06-28 22:22:06' AS DATE ), 'N' FROM DUAL
UNION ALL SELECT 1, 'A', CAST( TIMESTAMP '2015-06-28 22:25:11' AS DATE ), 'N' FROM DUAL
UNION ALL SELECT 1, 'B', CAST( TIMESTAMP '2015-06-28 22:29:44' AS DATE ), 'Y' FROM DUAL
UNION ALL SELECT 1, 'A', CAST( TIMESTAMP '2015-06-28 22:33:33' AS DATE ), 'N' FROM DUAL
UNION ALL SELECT 1, 'B', CAST( TIMESTAMP '2015-06-28 22:35:21' AS DATE ), 'N' FROM DUAL
UNION ALL SELECT 1, 'B', CAST( TIMESTAMP '2015-06-28 22:39:34' AS DATE ), 'Y' FROM DUAL
UNION ALL SELECT 1, 'B', CAST( TIMESTAMP '2015-06-28 22:43:53' AS DATE ), 'N' FROM DUAL
UNION ALL SELECT 1, 'A', CAST( TIMESTAMP '2015-06-28 22:45:53' AS DATE ), 'N' FROM DUAL
Query 1:
SELECT Key,
type,
timeStamp,
flag
FROM (
SELECT Key,
type,
timeStamp,
flag,
LAG( CASE WHEN type = 'B' THEN flag END ) IGNORE NULLS OVER ( PARTITION BY Key ORDER BY timeStamp ) AS prev_b_flag
FROM table_name t
WHERE type IN ( 'A', 'B' )
)
WHERE type = 'A'
AND flag = 'N'
AND prev_b_flag = 'Y'
Results:
| KEY | TYPE | TIMESTAMP | FLAG |
|-----|------|------------------------|------|
| 1 | A | June, 28 2015 22:25:11 | N |
| 1 | A | June, 28 2015 22:33:33 | N |

Answer 2

SELECT
*
FROM
tab A
WHERE
flag = 'N' AND type = 'A'
AND EXISTS (
SELECT
NULL
FROM
tab B
WHERE
type = 'B'
AND A.timestamp > timestamp AND A.Key = Key
GROUP BY
Key
HAVING
MAX(flag) KEEP (DENSE_RANK LAST ORDER BY timestamp) = 'Y'
);
There is no need to make correlated query to select flag from the the last record. Using aggregate KEEP clause is more efficient way. In this case it sort the groups by timestamp and keeps only the last value for the aggregation (last timestamp you wanted), so there comes only single record to the MAX function and we just take the FLAG value from it.
Here is simple example:
WITH sample (value1, value2) AS (
SELECT 1, 'Y' FROM DUAL UNION ALL
SELECT 2, 'X' FROM DUAL
)
SELECT
MIN(value2) KEEP (DENSE_RANK LAST ORDER BY value1) value2
FROM
sample
This returns value2 from the record with highest value1.