Hi all, thanks for replies. I have updated my query the below way.

SELECT

S_ID,

LAG(S_ACTV_CODE, 1) OVER (PARTITION BY S_ID ORDER BY S_DATETIME ASC) AS S_ACTV_CODE_PREV,

S_ACTV_CODE,

S_USER,

S_DATETIME AS START_TIME,

LEAD(S_DATETIME, 1) OVER (PARTITION BY S_ID ORDER BY S_DATETIME ASC) AS END_TIME,

DATEDIFF (SECOND, S_DATETIME,

LEAD(S_DATETIME, 1) OVER (PARTITION BY S_ID ORDER BY S_DATETIME ASC)) AS DURATION

FROM (

SELECT

S_ID,

LAG(S_ACTV_CODE, 1, 'N/A') OVER (PARTITION BY S_ID ORDER BY S_DATETIME ASC) AS S_ACTV_CODE_PREV,

S_ACTV_CODE,

S_USER,

S_DATETIME

FROM TABLE1

) X WHERE ISNULL(S_ACTV_CODE_PREV,'') != ISNULL(S_ACTV_CODE,'')

ORDER BY S_ID, START_TIME;

The above query worked for me and gave the expected results. Thanks once again for all your replies.