LAG looks at the row before

LAG hands you a value from the previous row, so each row can be compared with the one just before it — this month against last month. ORDER BY inside OVER decides what previous means. The very first row has nothing before it, so its LAG value is NULL.

A tiny example

In plain English: Show me each term's score next to the score from the term before it.

SELECT term, score, LAG(score) OVER (ORDER BY term) AS previous
FROM results;

What comes back

term	score	previous
Autumn	85	NULL
Spring	80	85
Winter	90	80

Each row's previous column is the score from the row above it — Autumn is first, so there's nothing before it.

Related SQL lessons

A query can hold another query inside itA query can hold another query inside it
EXISTS asks 'is there at least one?'EXISTS asks 'is there at least one?'
CASECASE turns values into labels
WITH starts a common table expression (CTE)WITH starts a common table expression (CTE)

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