Time zones and AT TIME ZONE in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to convert the current timestamp to UTC time zone.

PostgreSQL

SELECT CURRENT_TIMESTAMP [1] 'UTC';

Drag options to blanks, or click blank then click option'

AAT TIME ZONE

BNOW()

CTO_TIMESTAMP

DEXTRACT

Attempts:

3 left

💡 Hint

Common Mistakes

Using NOW() instead of AT TIME ZONE.

Trying to use EXTRACT for time zone conversion.

The AT TIME ZONE clause converts a timestamp to the specified time zone.

2fill in blank

medium

Complete the code to convert a timestamp with time zone to 'America/New_York' time zone.

PostgreSQL

SELECT timestamp_with_time_zone_column [1] 'America/New_York' FROM events;

Drag options to blanks, or click blank then click option'

ADATE_TRUNC

BAT TIME ZONE

CEXTRACT

DTO_CHAR

Attempts:

3 left

💡 Hint

Common Mistakes

Using TO_CHAR which formats but does not convert time zones.

Using EXTRACT which extracts parts of the timestamp.

AT TIME ZONE converts the timestamp to the specified time zone.

3fill in blank

hard

Fix the error in the code to convert '2024-06-01 12:00:00' timestamp to 'Europe/London' time zone.

PostgreSQL

SELECT TIMESTAMP '2024-06-01 12:00:00' [1] 'Europe/London';

Drag options to blanks, or click blank then click option'

ACONVERT_TZ

BTO_TIMESTAMP

CAT TIME ZONE

DSET TIMEZONE

Attempts:

3 left

💡 Hint

Common Mistakes

Using CONVERT_TZ which is MySQL syntax.

Using SET TIMEZONE which sets session time zone but does not convert timestamp.

AT TIME ZONE is the correct syntax to convert timestamp to a time zone in PostgreSQL.

4fill in blank

hard

Fill both blanks to convert a timestamp without time zone to UTC and then to 'Asia/Tokyo'.

PostgreSQL

SELECT (timestamp_column [1] 'UTC') [2] 'Asia/Tokyo' FROM meetings;

Drag options to blanks, or click blank then click option'

AAT TIME ZONE

BTO_TIMESTAMP

CNOW()

DEXTRACT

Attempts:

3 left

💡 Hint

Common Mistakes

Using TO_TIMESTAMP which parses strings but does not convert time zones.

Using NOW() which returns current time, not conversion.

First, convert the timestamp without time zone to UTC using AT TIME ZONE 'UTC', then convert that result to 'Asia/Tokyo' time zone again using AT TIME ZONE 'Asia/Tokyo'.

5fill in blank

hard

Fill all three blanks to convert 'event_time' from 'UTC' to 'Europe/Paris' and extract the hour.

PostgreSQL

SELECT EXTRACT(HOUR FROM (event_time [1] [2]) [3] 'Europe/Paris') AS paris_hour FROM schedule;

Drag options to blanks, or click blank then click option'

AAT TIME ZONE

B'UTC'

D'Europe/Paris'

Attempts:

3 left

💡 Hint

Common Mistakes

Using only one AT TIME ZONE clause.

Not quoting the time zone names.

First, convert event_time from 'UTC' using AT TIME ZONE 'UTC', then convert that timestamp to 'Europe/Paris' using AT TIME ZONE 'Europe/Paris', finally extract the hour.

Practice

1. What does the AT TIME ZONE clause do in PostgreSQL?

easy

A. It converts a timestamp from one time zone to another.

B. It changes the system time zone permanently.

C. It formats the timestamp as a string.

D. It deletes time zone information from a timestamp.

Solution

1. Step 1: Understand the purpose of AT TIME ZONE

   This clause is used to convert timestamps between different time zones.

2. Step 2: Compare options

   Only It converts a timestamp from one time zone to another. correctly describes this conversion. Other options describe unrelated actions.

3. Final Answer:

   It converts a timestamp from one time zone to another. -> Option A

4. Quick Check:

   AT TIME ZONE converts time zones [OK]

Hint: Remember: AT TIME ZONE changes time zones, not system settings [OK]

Common Mistakes:

• Thinking it changes system time zone permanently
• Confusing it with formatting functions
• Assuming it removes time zone info

2. Which of the following is the correct syntax to convert a timestamp to 'America/New_York' time zone in PostgreSQL?

easy

A. SELECT timestamp_column AT TIMEZONE 'America/New_York';

B. SELECT timestamp_column AT TIME ZONE 'America/New_York';

C. SELECT timestamp_column AT TIMEZONE America/New_York;

D. SELECT timestamp_column AT TIME ZONE America/New_York;

Solution

1. Step 1: Recall correct syntax for AT TIME ZONE

   The correct syntax uses two words: AT TIME ZONE followed by the time zone in single quotes.

2. Step 2: Check each option

   Only SELECT timestamp_column AT TIME ZONE 'America/New_York'; matches the correct syntax exactly. The others either use AT TIMEZONE as one word (incorrect spelling) or miss quotes around the time zone name.

3. Final Answer:

   SELECT timestamp_column AT TIME ZONE 'America/New_York'; -> Option B

4. Quick Check:

   Use AT TIME ZONE 'zone' with quotes [OK]

Hint: Use two words: AT TIME ZONE with quotes around zone [OK]

Common Mistakes:

• Writing AT TIMEZONE as one word
• Omitting quotes around time zone
• Using incorrect keyword order

3. Given the query:

SELECT '2024-06-01 12:00:00'::timestamp AT TIME ZONE 'UTC' AT TIME ZONE 'America/Los_Angeles';

What is the output timestamp?

medium

A. 2024-06-01 20:00:00

B. 2024-06-01 04:00:00

C. 2024-06-01 05:00:00

D. 2024-06-01 12:00:00

Solution

1. Step 1: Understand the query steps

   The first AT TIME ZONE 'UTC' converts the timestamp without time zone to UTC timestamp with time zone. The second AT TIME ZONE 'America/Los_Angeles' converts that UTC time to Los Angeles local time.

2. Step 2: Calculate time difference

   On June 1, Los Angeles is on PDT (UTC-7). So 12:00 UTC minus 7 hours is 05:00 local time. But since the query applies AT TIME ZONE twice, the final output is a timestamp without time zone in Los Angeles time, which is 05:00.

3. Final Answer:

   2024-06-01 05:00:00 -> Option C

4. Quick Check:

   UTC 12:00 minus 7 hours = 05:00 [OK]

Hint: Remember LA is UTC-7 in June; subtract 7 hours [OK]

Common Mistakes:

• Confusing the order of AT TIME ZONE conversions
• Ignoring daylight saving time offset
• Assuming output is in UTC

4. Identify the error in this query:

SELECT '2024-06-01 12:00:00'::timestamp AT TIMEZONE 'Europe/London';

medium

A. The timestamp literal is missing time zone info.

B. The query is correct and will run without error.

C. The time zone name is invalid.

D. The keyword should be AT TIME ZONE with a space.

Solution

1. Step 1: Check the syntax of AT TIME ZONE

   The query uses AT TIMEZONE as one word, which is incorrect syntax in PostgreSQL.

2. Step 2: Verify other parts

   The timestamp literal is valid, and 'Europe/London' is a valid time zone name. So no error there.

3. Final Answer:

   The keyword should be AT TIME ZONE with a space. -> Option D

4. Quick Check:

   Use two words: AT TIME ZONE [OK]

Hint: Always write AT TIME ZONE as two words [OK]

Common Mistakes:

• Writing AT TIMEZONE as one word
• Assuming timestamp needs time zone info
• Thinking time zone name is invalid

5. You have a timestamp with time zone column created_at stored in UTC. You want to display it as local time in 'Asia/Tokyo' but keep it as timestamp without time zone. Which query correctly does this?

hard

A. SELECT created_at AT TIME ZONE 'Asia/Tokyo' FROM orders;

B. SELECT created_at::timestamp AT TIME ZONE 'Asia/Tokyo' FROM orders;

C. SELECT (created_at AT TIME ZONE 'UTC') AT TIME ZONE 'Asia/Tokyo' FROM orders;

D. SELECT created_at AT TIME ZONE 'UTC' AT TIME ZONE 'Asia/Tokyo' FROM orders;

Solution

1. Step 1: Understand data type and conversion

   created_at is timestamp with time zone in UTC. Applying AT TIME ZONE 'Asia/Tokyo' converts it to timestamp without time zone in Tokyo local time.

2. Step 2: Analyze options

   SELECT created_at AT TIME ZONE 'Asia/Tokyo' FROM orders; correctly applies AT TIME ZONE 'Asia/Tokyo' to convert UTC timestamptz to local timestamp. SELECT created_at::timestamp AT TIME ZONE 'Asia/Tokyo' FROM orders; casts to timestamp first, losing time zone info before conversion, causing wrong result. The options that apply AT TIME ZONE 'UTC' first perform extra and incorrect conversions.

3. Final Answer:

   SELECT created_at AT TIME ZONE 'Asia/Tokyo' FROM orders; -> Option A

4. Quick Check:

   timestamptz AT TIME ZONE zone = local timestamp [OK]

Hint: Apply AT TIME ZONE directly on timestamptz to get local timestamp [OK]

Common Mistakes:

• Casting timestamptz to timestamp before conversion
• Applying multiple AT TIME ZONE conversions unnecessarily
• Confusing timestamp with and without time zone