Querying historical data (AT, BEFORE) in Snowflake - Mini Project: Build & Apply

Querying historical data (AT, BEFORE)

📖 Scenario: You work as a data analyst for a retail company. The company uses Snowflake to store sales data. You want to learn how to query the sales table to see what the data looked like at a specific point in time or before that time.

🎯 Goal: Build a Snowflake SQL query that retrieves sales data as it existed at a specific timestamp and before a specific timestamp using the AT and BEFORE clauses.

📋 What You'll Learn

Create a table called sales with columns sale_id (integer), product (string), and amount (number).

Insert sample sales data with at least 3 rows.

Write a query to select all sales data AT a specific timestamp.

Write a query to select all sales data BEFORE a specific timestamp.

💡 Why This Matters

🌍 Real World

Companies often need to see how their data looked at a past point in time to audit changes or recover lost data.

💼 Career

Knowing how to query historical data is important for data analysts and engineers working with Snowflake or other cloud data warehouses.

1

Create the sales table and insert sample data

Create a table called sales with columns sale_id as integer, product as string, and amount as number. Then insert these exact rows: (1, 'Shoes', 50), (2, 'Hat', 20), (3, 'Jacket', 100).

Snowflake

-- Create the sales table and insert sample data
-- Your code here

Hint

Use CREATE OR REPLACE TABLE to create the table and INSERT INTO to add rows.

2

Set a timestamp variable for querying historical data

Create a variable called query_time and set it to the timestamp '2024-01-01 10:00:00' to use for querying historical data.

Snowflake

CREATE OR REPLACE TABLE sales (
  sale_id INT,
  product STRING,
  amount NUMBER
);

INSERT INTO sales (sale_id, product, amount) VALUES
  (1, 'Shoes', 50),
  (2, 'Hat', 20),
  (3, 'Jacket', 100);

-- Create a variable called query_time with the timestamp '2024-01-01 10:00:00'
-- Your code here

Hint

Use SET to create a session variable and TO_TIMESTAMP to convert the string to a timestamp.

3

Write a query to select sales data AT the timestamp

Write a query that selects all columns from sales AT the timestamp stored in query_time.

Snowflake

CREATE OR REPLACE TABLE sales (
  sale_id INT,
  product STRING,
  amount NUMBER
);

INSERT INTO sales (sale_id, product, amount) VALUES
  (1, 'Shoes', 50),
  (2, 'Hat', 20),
  (3, 'Jacket', 100);

SET query_time = TO_TIMESTAMP('2024-01-01 10:00:00');

-- Write a query to select all columns from sales AT the timestamp query_time
-- Your code here

Hint

Use AT (TIMESTAMP => $query_time) after the table name to query data at that time.

4

Write a query to select sales data BEFORE the timestamp

Write a query that selects all columns from sales BEFORE the timestamp stored in query_time.

Snowflake

CREATE OR REPLACE TABLE sales (
  sale_id INT,
  product STRING,
  amount NUMBER
);

INSERT INTO sales (sale_id, product, amount) VALUES
  (1, 'Shoes', 50),
  (2, 'Hat', 20),
  (3, 'Jacket', 100);

SET query_time = TO_TIMESTAMP('2024-01-01 10:00:00');

SELECT * FROM sales AT (TIMESTAMP => $query_time);

-- Write a query to select all columns from sales BEFORE the timestamp query_time
-- Your code here

Hint

Use BEFORE (TIMESTAMP => $query_time) after the table name to query data before that time.

Practice

1. What does the AT clause do when querying historical data in Snowflake?

easy

A. Shows data exactly at a specified point in time

B. Shows data after a specified point in time

C. Shows data before a specified point in time

D. Shows the latest data ignoring time

Solution

1. Step 1: Understand the purpose of AT clause

   The AT clause is used to query data as it existed exactly at a specific timestamp.

2. Step 2: Compare with other time clauses

   Unlike BEFORE, which shows data just before a time, AT shows the exact snapshot at that time.

3. Final Answer:

   Shows data exactly at a specified point in time -> Option A

4. Quick Check:

   AT = exact time snapshot [OK]

Hint: Remember: AT means exact time snapshot, not before or after [OK]

Common Mistakes:

• Confusing AT with BEFORE clause
• Thinking AT shows data after the time
• Assuming AT ignores time and shows latest data

2. Which of the following is the correct syntax to query a table named sales as it was exactly at timestamp '2024-01-01 10:00:00' using Snowflake's time travel feature?

easy

A. SELECT * FROM sales AT '2024-01-01 10:00:00';

B. SELECT * FROM sales AT (TIMESTAMP => '2024-01-01 10:00:00'; )

C. SELECT * FROM sales AT (TIMESTAMP => '2024-01-01 10:00:00')

D. SELECT * FROM sales BEFORE (TIMESTAMP => '2024-01-01 10:00:00');

Solution

1. Step 1: Identify correct syntax for AT clause

   Snowflake requires the AT clause with parentheses and the keyword TIMESTAMP => for time travel queries.

2. Step 2: Check syntax details

   SELECT * FROM sales AT (TIMESTAMP => '2024-01-01 10:00:00') correctly uses AT (TIMESTAMP => '2024-01-01 10:00:00') without a trailing semicolon inside the query string.

3. Final Answer:

   SELECT * FROM sales AT (TIMESTAMP => '2024-01-01 10:00:00') -> Option C

4. Quick Check:

   Correct AT syntax = SELECT * FROM sales AT (TIMESTAMP => '2024-01-01 10:00:00') [OK]

Hint: Use AT (TIMESTAMP => 'timestamp') with parentheses for time travel [OK]

Common Mistakes:

• Using BEFORE instead of AT for exact time
• Omitting parentheses around TIMESTAMP clause
• Adding semicolon inside query string causing syntax error

3. Given the table orders with changes over time, what will the following query return?

SELECT COUNT(*) FROM orders BEFORE (TIMESTAMP => '2024-03-01 12:00:00');

medium

A. Count of rows just before '2024-03-01 12:00:00'

B. Count of rows after '2024-03-01 12:00:00'

C. Count of rows exactly at '2024-03-01 12:00:00'

D. Count of all current rows ignoring time

Solution

1. Step 1: Understand BEFORE clause behavior

   The BEFORE clause returns data as it existed just before the specified timestamp.

2. Step 2: Apply to COUNT query

   The query counts rows in orders table as they were immediately before '2024-03-01 12:00:00'.

3. Final Answer:

   Count of rows just before '2024-03-01 12:00:00' -> Option A

4. Quick Check:

   BEFORE = data just before time [OK]

Hint: BEFORE means data snapshot just before the timestamp [OK]

Common Mistakes:

• Confusing BEFORE with AT clause
• Assuming BEFORE returns data after the timestamp
• Thinking BEFORE returns current data

4. You wrote this query but got a syntax error:

SELECT * FROM customers AT TIMESTAMP => '2024-05-01 09:00:00';

What is the likely cause?

medium

A. Table name should be in quotes

B. Missing parentheses around the TIMESTAMP clause

C. Timestamp format is incorrect

D. Using BEFORE instead of AT

Solution

1. Step 1: Check syntax for AT clause

   The AT clause requires parentheses around the TIMESTAMP => '...' part.

2. Step 2: Identify missing parentheses

   The query misses parentheses, causing a syntax error.

3. Final Answer:

   Missing parentheses around the TIMESTAMP clause -> Option B

4. Quick Check:

   AT clause needs parentheses [OK]

Hint: Always wrap TIMESTAMP clause in parentheses after AT [OK]

Common Mistakes:

• Omitting parentheses around TIMESTAMP
• Confusing syntax with BEFORE clause
• Incorrect timestamp format causing error

5. You want to compare the number of rows in the transactions table exactly at '2024-04-15 08:00:00' and just before that time. Which pair of queries correctly achieves this?

hard

A. SELECT COUNT(*) FROM transactions BEFORE (TIMESTAMP => '2024-04-15 08:00:00');SELECT COUNT(*) FROM transactions AT TIMESTAMP => '2024-04-15 08:00:00');

B. SELECT COUNT(*) FROM transactions BEFORE (TIMESTAMP => '2024-04-15 08:00:00');SELECT COUNT(*) FROM transactions AFTER (TIMESTAMP => '2024-04-15 08:00:00');

C. SELECT COUNT(*) FROM transactions AT (TIMESTAMP => '2024-04-15 08:00:00');SELECT COUNT(*) FROM transactions AT (TIMESTAMP => '2024-04-15 07:59:59');

D. SELECT COUNT(*) FROM transactions AT (TIMESTAMP => '2024-04-15 08:00:00');SELECT COUNT(*) FROM transactions BEFORE (TIMESTAMP => '2024-04-15 08:00:00');

Solution

1. Step 1: Identify query for exact time

   Use AT (TIMESTAMP => '2024-04-15 08:00:00') to get data exactly at that timestamp.

2. Step 2: Identify query for just before time

   Use BEFORE (TIMESTAMP => '2024-04-15 08:00:00') to get data just before that timestamp.

3. Step 3: Verify option matches both queries

   SELECT COUNT(*) FROM transactions AT (TIMESTAMP => '2024-04-15 08:00:00');
   SELECT COUNT(*) FROM transactions BEFORE (TIMESTAMP => '2024-04-15 08:00:00'); correctly uses AT and BEFORE clauses as required.

4. Final Answer:

   SELECT COUNT(*) FROM transactions AT (TIMESTAMP => '2024-04-15 08:00:00');
   SELECT COUNT(*) FROM transactions BEFORE (TIMESTAMP => '2024-04-15 08:00:00'); -> Option D

5. Quick Check:

   AT = exact, BEFORE = just before [OK]

Hint: Use AT for exact time, BEFORE for just before time [OK]

Common Mistakes:

• Swapping AT and BEFORE clauses
• Using AFTER which is invalid in Snowflake time travel
• Using two AT queries with different timestamps