What Is an ASOF Join?

An ASOF (or “as of”) join is a type of join operation used when analyzing two sets of time-series data. It essentially matches each record from one table with the nearest—but not necessarily equal—value from another table based on a chosen column. Snowflake supports this out of the box using using the ASOF keyword

The chosen column needs to have some concept of range for the ASOF operation to work. You may think of it as being the “closest value,” but not exceeding the comparison. In Snowflake it works for Timestamp and Numeric valued columns.

Understanding ASOF Joins

ASOF joins are a powerful tool when dealing with time-series data. In simple terms, an ASOF join will, for each row in the left table, find a corresponding single row in the right table where the key value is less than or equal to the key in the left table.

This is a common operation when dealing with financial data, sensor readings, or other types of time-series data where readings might not align perfectly by timestamp.

For a simple example, consider the real-world question, “What was the temperature yesterday at this time?” It is very unlikely that a temperature reading was taken yesterday at exactly the millisecond that the question is asked today. What we really want is “What was the temperature taken yesterday up to today’s time stamp?”

This simple example becomes a lot more complex when we start comparing temperatures day over day, week over week, etc.

ASOF Joins in Snowflake

Step 1: Prepare your data

Ensure your data is in the correct format for the ASOF join. You’ll need a timestamp or other monotonically increasing column to use as a key for the join.

Suppose you have two tables, stock_prices and stock_portfolio, each containing a timestamp column, and you want to join them by stock and the nearest timestamp.

stock_prices

SYMBOL PRICE STOCK_PRICE_ASOF
TSLA 9.93 2024-02-02 14:29:10.753
TSLA 9.62 2024-02-02 14:29:52.806
TSLA 9.61 2024-02-02 14:30:43.379
TSLA 9.93 2024-02-02 14:32:35.397
TSLA 9.68 2024-02-02 14:41:38.263
TSLA 9.05 2024-02-02 14:45:00.480
TSLA 9.06 2024-02-02 14:45:32.556
TSLA 9.88 2024-02-02 15:04:29.979
TSLA 9.72 2024-02-02 15:06:40.404
TSLA 9.44 2024-02-02 15:23:07.893
TSLA 9.13 2024-02-02 15:23:21.083

stock_portfolio

SYMBOL NUMBER_OF_SHARES TRANSACTION_TIMESTAMP
TSLA 100.00 2024-02-02 14:29:20.264
TSLA 77.00 2024-02-02 14:30:51.040
TSLA 56.00 2024-02-02 14:45:41.192
TSLA 86.00 2024-02-02 15:06:48.475
TSLA 60.00 2024-02-02 15:23:12.587

Note that the transaction_timestamp from stock_portfolio does not align with the stock_price_asof in the stock_prices table.

Step 2: Query the data using a ASOF join

select 
  stock_portfolio.symbol
  , price as stock_price
  , number_of_shares
  , transaction_timestamp
  , stock_price_asof  
  , price * number_of_shares as value
from stock_portfolio
asof join stock_prices
  match_condition (stock_portfolio.transaction_timestamp >= stock_prices.stock_price_asof)
  on stock_prices.symbol = stock_portfolio.symbol
;

This attaches the value of the holding at that time to each row.

SYMBOL STOCK_PRICE NUMBER_OF_SHARES TRANSACTION_TIMESTAMP STOCK_PRICE_ASOF VALUE
TSLA 9.93 100.00 2024-02-02 14:29:20.264 2024-02-02 14:29:10.753 993.0000
TSLA 9.61 77.00 2024-02-02 14:30:51.040 2024-02-02 14:30:43.379 739.9700
TSLA 9.06 56.00 2024-02-02 14:45:41.192 2024-02-02 14:45:32.556 507.3600
TSLA 9.72 86.00 2024-02-02 15:06:48.475 2024-02-02 15:06:40.404 835.9200
TSLA 9.44 60.00 2024-02-02 15:23:12.587 2024-02-02 15:23:07.893 566.4000

It essentially executes a function defined by looking up nearby values in the stock_prices table.

See also