Querying historical data (AT, BEFORE) in Snowflake - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the AT clause do in Snowflake time travel queries?

The AT clause lets you query the exact state of a table at a specific point in time in the past.

intermediate

How does the BEFORE clause differ from AT in Snowflake?

BEFORE returns the state of the data just before a specified time or query ID, while AT returns the state exactly at that time.

beginner

What is the maximum time travel retention period in Snowflake by default?

By default, Snowflake retains historical data for 1 day (24 hours) for time travel queries.

intermediate

Write a simple Snowflake query to get the state of a table named orders exactly 2 hours ago.

SELECT * FROM orders AT (TIMESTAMP => DATEADD(hour, -2, CURRENT_TIMESTAMP()));

beginner

Why is querying historical data useful in cloud data warehouses like Snowflake?

It helps recover lost or changed data, audit changes, and analyze past states without restoring backups.

What does the AT clause in Snowflake time travel do?

A Deletes data at a specified time

B Shows data before a specified past time

C Shows current data only

D Shows data exactly at a specified past time

AT returns the exact state of data at a given past timestamp.

Which clause returns data just before a specified time in Snowflake?

A AT

B SINCE

C BEFORE

D AFTER

BEFORE returns the state of data immediately before the specified time.

What is the default time travel retention period in Snowflake?

A 12 hours

B 24 hours

C 1 hour

D 7 days

Snowflake retains historical data for 24 hours by default.

Which of these is a valid Snowflake query to get data 30 minutes ago?

A SELECT * FROM table AT (TIMESTAMP => DATEADD(minute, -30, CURRENT_TIMESTAMP()));

B SELECT * FROM table BEFORE (TIMESTAMP => DATEADD(minute, -30, CURRENT_TIMESTAMP()));

C SELECT * FROM table SINCE (TIMESTAMP => DATEADD(minute, -30, CURRENT_TIMESTAMP()));

D SELECT * FROM table AFTER (TIMESTAMP => DATEADD(minute, -30, CURRENT_TIMESTAMP()));

AT with DATEADD gets the exact state at 30 minutes ago.

Why use Snowflake time travel to query historical data?

A To recover or audit past data states

B To delete old data

C To speed up current queries

D To create new tables

Time travel helps recover or audit data as it was in the past.

Explain how you would use the AT and BEFORE clauses in Snowflake to query historical data.

Think about recovering data or auditing changes.

Describe a real-life scenario where querying historical data with Snowflake time travel would be helpful.

Imagine you lost some important data or want to check what changed.

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