Take the row after the specific row - sql

Question

I have the table, where I need to take the next row after the row which has course 'TA' and flag = 1. For this I created the column rnum (OVER DATE) which may help for finding it
| student | date | course | flag | rnum |
| ------- | ----- | ----------- | ---- | ---- |
| 1 | 17:00 | Math | null | 1 |
| 1 | 17:10 | Python | null | 2 |
| 1 | 17:15 | TA | 1 | 3 |
| 1 | 17:20 | English | null | 4 |
| 1 | 17:35 | Geography | null | 5 |
| 2 | 16:10 | English | null | 1 |
| 2 | 16:20 | TA | 1 | 2 |
| 2 | 16:30 | SQL | null | 3 |
| 2 | 16:40 | Python | null | 4 |
| 3 | 19:05 | English | null | 1 |
| 3 | 19:20 | Literachure | null | 2 |
| 3 | 19:30 | TA | null | 3 |
| 3 | 19:40 | Python | null | 4 |
| 3 | 19:50 | Python | null | 5 |
As a result I should have:
| student | date | course | flag | rnum |
| ------- | ----- | ------- | ---- | ---- |
| 1 | 17:20 | English | null | 4 |
| 2 | 16:30 | SQL | null | 3 |

Answer 1

There are many ways to get your desired result, let's see some of them.
1) EXISTS
You can use the EXISTS clause, specifying a subquery to match for the condition.
SELECT T2.*
FROM #MyTable T2
WHERE EXISTS (
SELECT 'x' x
FROM #MyTable T1
WHERE T1.course = 'TA' AND T1.flag = 1
AND T1.student = T2.student AND T2.rnum = T1.rnum + 1
)
2) LAG
You ca use window function LAG to access previous row for a given order and then filter your resultset with your conditions.
SELECT w.student, w.date, w.course, w.flag, w.rnum
FROM (
SELECT T1.*
, LAG(course, 1) OVER (PARTITION BY student ORDER BY rnum) prevCourse
, LAG(flag, 1) OVER (PARTITION BY student ORDER BY rnum) prevFlag
FROM #MyTable T1
) w
WHERE prevCourse = 'TA' AND prevFlag = 1
3) JOIN
You can self-JOIN your table on the next rnum and keep only the rows who match the right condition.
SELECT T2.*
FROM MyTable T1
JOIN MyTable T2 ON T1.student = T2.student AND T2.rnum = T1.rnum + 1
WHERE T1.course = 'TA' AND T1.flag = 1
4) CROSS APPLY
You can use CROSS APPLY to specify a subquery with the matching condition. It is pretty similar to EXISTS clause, but you will also get in your resultset the columns from the subquery.
SELECT T2.*
FROM #MyTable T2
CROSS APPLY (
SELECT 'x' x
FROM #MyTable T1
WHERE T1.course = 'TA' AND T1.flag = 1
AND T1.student = T2.student AND T2.rnum = T1.rnum + 1
) x
5) CTE
You can use common table expression (CTE) to extract matching rows and then use it to filter your table with a JOIN.
;WITH
T1 AS (
SELECT student, rnum
FROM #MyTable T1
WHERE T1.course = 'TA' AND T1.flag = 1
)
SELECT T2.*
FROM #MyTable T2
JOIN T1 ON T1.student = T2.student AND T2.rnum = T1.rnum + 1

Answer 2

Adding the rownumber was a good start, you can use it to join the table with itself:
WITH matches AS (
SELECT
student,
rnum
FROM table
WHERE flag = 1
AND course = 'TA'
)
SELECT t.*
FROM table t
JOIN matches m
on t.student = m.student
and t.rnum = m.rnum + 1