Rising Temperature
You can write a query to find the dates’ Id with higher temperatures compared to its previous dates. Here’s the SQL code:
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This query joins the Weather table with itself, matching rows where the recordDate is one day apart. It then filters the results to only include rows where the temperature is higher on the later date.
The ON
keyword is used to specify the condition that must be met for the join to occur between two rows from the two instances of the Weather table (aliased as w1
and w2
).
Here’s the condition:
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The DATEDIFF
function calculates the difference in days between two dates. In this case, it’s finding the difference between the recordDate
column in the w1
alias and the recordDate
column in the w2
alias.
The condition DATEDIFF(w1.recordDate, w2.recordDate) = 1
ensures that only rows where the recordDate
in w1
is one day later than the recordDate
in w2
will be joined.
So essentially, the ON
clause is used to pair each row in the table with the row representing the previous day, allowing the query to then compare the temperatures between consecutive days.
In the context of the query, the statement within the ON
clause is used to figure out the “yesterday” for each given date in the Weather table.
By using the condition DATEDIFF(w1.recordDate, w2.recordDate) = 1
, the query is looking for pairs of rows where the difference in the recordDate
column is exactly one day. In this pairing, w1.recordDate
represents the current day, and w2.recordDate
represents the previous day, or “yesterday.”
This condition ensures that the query only considers pairs of dates that are consecutive, allowing it to identify the dates with higher temperatures compared to the previous day.