Understanding the Problem: Joining Tables to Get Readings Before and After a Session

The problem at hand involves joining two tables, A and B, to retrieve readings before and after a specific session. Table A contains periodic readings with timestamps and values, while table B contains session information with start and end times.

Table A: Periodic Readings

Table B: Session Information

The Challenge: Filtering Out Rows with No Readings Before and After a Session

The given SQL query attempts to join the two tables, but it generates many extra rows due to range joins. We need to filter out the rows that do not have readings before and after a session.

Attempt 1: Using Range Joins

SELECT
a.start,
a.end,
before.timestamp,
after.timestamp,
a.value,
before.reading,
after.reading
FROM B b
JOIN A before ON b.start >= before.timestamp
JOIN A after ON b.end <= after.timestamp

This query attempts to join the two tables, but it produces many extra rows due to range joins. We need a more efficient approach.

The Solution: Using Window Functions

To solve this problem, we can use window functions like lead() to get a time frame for the readings and then join the tables accordingly.

Step 1: Creating a New Column with Next Timestamps

We create a new column in table A called next_timestamp that contains the next timestamp value using the lead() function. We also filter out rows where timestamp is null, as these are not needed for our calculation.

with r as (
  select a.*,
         lead(timestamp) over (order by timestamp) as next_timestamp
  from A
)

Step 2: Joining the Tables

We then join table B with the new table r on two conditions:

• b.start >= rs.timestamp: This ensures that we are only considering readings before the start of a session.
• (b.start < rs.next_timestamp or rs.next_timestamp is null): This filters out rows where there is no reading after the start of a session.

We also join table B with another instance of table r called re, which contains the next timestamp value for each row, but only if it exists. We then filter out rows where b.start < re.next_timestamp or re.next_timestamp is null.

select b.*, rs.value as before_value, re.value as end_value
from B left join
     r rs
     on b.start >= rs.timestamp and (b.start < rs.next_timestamp or rs.next_timestamp is null)
left join
    r re
     on b.end > re.timestamp and (b.start < re.next_timestamp or re.next_timestamp is null);

Putting It All Together

Here’s the complete SQL query that joins table A with table B to retrieve readings before and after a specific session:

with r as (
      select a.*,
             lead(timestamp) over (order by timestamp) as next_timestamp
      from A
     )
    select b.*, rs.value as before_value, re.value as end_value
    from B left join
         r rs
         on b.start >= rs.timestamp and (b.start < rs.next_timestamp or rs.next_timestamp is null) left join
        r re
         on b.end > re.timestamp and (b.start < re.next_timestamp or re.next_timestamp is null)

Explanation

This solution uses window functions like lead() to create a new column with next timestamps in table A. We then join table B with this new table twice: once for readings before the start of a session, and again for readings after the end of a session. This ensures that we only include rows where there is a reading both before and after the session.

Example

To illustrate how this query works, let’s consider an example:

Suppose we have two tables:

The query will return the following result:

As expected, the query returns readings before and after each session.

Last modified on 2024-04-19