SQL Server LEAD function - sql

Question

-- FIRST LOGIN DATE
WITH CTE_FIRST_LOGIN AS
(
SELECT
PLAYER_ID, EVENT_DATE,
ROW_NUMBER() OVER (PARTITION BY PLAYER_ID ORDER BY EVENT_DATE ASC) AS RN
FROM
ACTIVITY
),
-- CONSECUTIVE LOGINS
CTE_CONSEC_PLAYERS AS
(
SELECT
PLAYER_ID,
LEAD(EVENT_DATE,1) OVER (PARTITION BY EVENT_DATE ORDER BY EVENT_DATE) NEXT_DATE
FROM
ACTIVITY A
JOIN
CTE_FIRST_LOGIN C ON A.PLAYER_ID = C.PLAYER_ID
WHERE
NEXT_DATE = DATEADD(DAY, 1, A.EVENT_DATE) AND C.RN = 1
GROUP BY
A.PLAYER_ID
)
-- FRACTION
SELECT
NULLIF(ROUND(1.00 * COUNT(CTE_CONSEC.PLAYER_ID) / COUNT(DISTINCT PLAYER_ID), 2), 0) AS FRACTION
FROM
ACTIVITY
JOIN
CTE_CONSEC_PLAYERS CTE_CONSEC ON CTE_CONSEC.PLAYER_ID = ACTIVITY.PLAYER_ID
I am getting the following error when I run this query.
[42S22] [Microsoft][ODBC Driver 17 for SQL Server][SQL Server]Invalid column name 'NEXT_DATE'. (207) (SQLExecDirectW)
This is a leetcode medium question 550. Game Play Analysis IV. I wanted to know why it can't identify the column NEXT_DATE here and what am I missing? Thanks!

Answer 1

The problem is in this CTE:
-- CONSECUTIVE LOGINS prep
CTE_CONSEC_PLAYERS AS (
SELECT
PLAYER_ID,
LEAD(EVENT_DATE,1) OVER (PARTITION BY EVENT_DATE ORDER BY EVENT_DATE) NEXT_DATE
FROM ACTIVITY A
JOIN CTE_FIRST_LOGIN C ON A.PLAYER_ID = C.PLAYER_ID
WHERE NEXT_DATE = DATEADD(DAY, 1, A.EVENT_DATE) AND C.RN = 1
GROUP BY A.PLAYER_ID
)
Note that you are creating NEXT_DATE as a column alias in this CTE but also referring to it in the WHERE clause. This is invalid because by SQL clause-ordering rules the NEXT_DATE column alias does not exist until you get to the ORDER BY clause which is the last evaluated clause in a SQL query or subquery. You don't have an ORDER BY clause in this subquery, so technically the NEXT_DATE column alias only exists to [sub]queries that both come after and reference your CTE_CONSEC_PLAYERS CTE.
To fix this you'd probably want two CTEs like this (untested):
-- CONSECUTIVE LOGINS
CTE_CONSEC_PLAYERS_pre AS (
SELECT
PLAYER_ID,
RN,
EVENT_DATE,
LEAD(EVENT_DATE,1) OVER (PARTITION BY EVENT_DATE ORDER BY EVENT_DATE) NEXT_DATE
FROM ACTIVITY A
JOIN CTE_FIRST_LOGIN C ON A.PLAYER_ID = C.PLAYER_ID
)
-- CONSECUTIVE LOGINS
CTE_CONSEC_PLAYERS AS (
SELECT
PLAYER_ID,
MAX(NEXT_DATE) AS NEXT_DATE,
FROM CTE_CONSEC_PLAYERS_pre
WHERE NEXT_DATE = DATEADD(DAY, 1, EVENT_DATE) AND RN = 1
GROUP BY PLAYER_ID
)

Answer 2

You gave every table an alias (for example JOIN CTE_FIRST_LOGIN C has the alias C), and every column access is via the alias. You need to add the correct alias from the correct table to NEXT_DATE.

Answer 3

Your primary issue is that NEXT_DATE is a window function, and therefore cannot be referred to in the WHERE because of SQL's order of operations.
But it seems this query is over-complicated.
The problem to be solved appears to be: how many players logged in the day after they first logged in, as a percentage of all players.
This can be done in a single pass (no joins), by using multiple window functions together:
WITH CTE_FIRST_LOGIN AS (
SELECT
PLAYER_ID,
EVENT_DATE,
ROW_NUMBER() OVER (PARTITION BY PLAYER_ID ORDER BY EVENT_DATE) AS RN,
-- if EVENT_DATE is a datetime and can have multiple per day then group by CAST(EVENT_DATE AS date) first
LEAD(EVENT_DATE, 1) OVER (PARTITION BY EVENT_DATE ORDER BY EVENT_DATE) AS NextDate
FROM ACTIVITY
),
BY_PLAYERS AS (
SELECT
c.PLAYER_ID,
SUM(CASE WHEN c.RN = 1 AND c.NextDate = DATEADD(DAY, 1, c.EVENT_DATE)
THEN 1 END) AS IsConsecutive
FROM CTE_FIRST_LOGIN AS c
GROUP BY c.PLAYER_ID
)
SELECT ROUND(
1.00 *
COUNT(c.IsConsecutive) /
NULLIF(COUNT(*), 0)
,2) AS FRACTION
FROM BY_PLAYERS AS c;
You could theoretically merge BY_PLAYERS into the outer query and use COUNT(DISTINCT but splitting them feels cleaner