Chapter 1 · Practice 2 of 4

Declare the grain in one sentence and keep every row at that grain

From Start from the business process and the grain

Why it matters

Write the grain as a sentence before any SQL: "one row per order line", "one row per product per day". Microsoft's fact table guidance explains that the values of the dimension keys set the granularity: a targets table whose date key always holds the first day of a month is at month level, even though the column is a date. Every row must follow the same rule.

A grain statement settles arguments early:

Table	Grain
`fct_order_lines`	One row per order line
`fct_daily_inventory`	One row per product per warehouse per day
`fct_sales_targets`	One row per salesperson per quarter

If a requirement needs a different level of detail, it usually needs a different fact table, not extra rows in the existing one. Put the grain sentence in the model's description so everyone downstream can read it.

Avoid

Adding summary rows such as "monthly total" to a table of individual events