TO_DATE and TO_TIMESTAMP for parsing in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the TO_DATE function do in PostgreSQL?

TO_DATE converts a string into a date type using a specified format.

It helps turn text like '2024-06-01' into a real date the database understands.

beginner

How is TO_TIMESTAMP different from TO_DATE?

TO_TIMESTAMP converts a string into a timestamp, including date and time parts.

TO_DATE only converts to a date without time.

beginner

Example: What does TO_DATE('31-12-2023', 'DD-MM-YYYY') return?

It returns the date 2023-12-31 as a date type.

The format 'DD-MM-YYYY' tells PostgreSQL how to read the string.

intermediate

Why do you need to specify a format string in TO_DATE or TO_TIMESTAMP?

The format string tells PostgreSQL how to read the parts of the date/time in the text.

Without it, PostgreSQL might misunderstand the order or meaning of day, month, year, hour, etc.

intermediate

What happens if the input string does not match the format in TO_DATE?

PostgreSQL will raise an error because it cannot correctly parse the string into a date.

Always ensure the string matches the format exactly.

Which function converts a string to a timestamp including time information?

A NOW()

B TO_DATE

C CAST

D TO_TIMESTAMP

TO_TIMESTAMP converts strings to timestamps with date and time.

What does TO_DATE('2024/06/01', 'YYYY/MM/DD') return?

A A timestamp value for June 1, 2024

B A date value for June 1, 2024

C An error because of wrong format

D A string '2024/06/01'

The function returns a date because the string matches the format.

If you want to parse '15:30:00' as time only, which function is best?

A TO_TIMESTAMP

B TO_DATE

C TO_TIME

D CAST

TO_TIMESTAMP can parse time parts; TO_DATE cannot.

What format string matches '31-12-2023'?

A MM-DD-YYYY

B YYYY-MM-DD

C DD-MM-YYYY

D YYYY-DD-MM

Day first, then month, then year matches '31-12-2023'.

What happens if the input string is '2023-31-12' but format is 'YYYY-MM-DD'?

A Error due to mismatch

B Parsed correctly

C Returns NULL

D Returns current date

Format and string must match exactly or an error occurs.

Explain how to use TO_DATE to convert a string into a date in PostgreSQL.

Think about how you tell PostgreSQL the order of day, month, and year.

Describe the difference between TO_DATE and TO_TIMESTAMP and when to use each.

Consider if you need hours and minutes or just the day.

Practice

1. What does the PostgreSQL function TO_DATE('2024-06-15', 'YYYY-MM-DD') return?

easy

A. A timestamp value with time set to midnight

B. A date value representing June 15, 2024

C. A string '2024-06-15'

D. An error because format is incorrect

Solution

1. Step 1: Understand TO_DATE function

   TO_DATE converts a text string to a date type using the given format.

2. Step 2: Match input and format

   The string '2024-06-15' matches the format 'YYYY-MM-DD', so it converts correctly to a date.

3. Final Answer:

   A date value representing June 15, 2024 -> Option B

4. Quick Check:

   TO_DATE returns date type [OK]

Hint: TO_DATE returns date, TO_TIMESTAMP returns timestamp [OK]

Common Mistakes:

• Confusing TO_DATE with TO_TIMESTAMP
• Using wrong format string causing errors
• Expecting string output instead of date

2. Which of the following is the correct syntax to convert the string '15/06/2024 14:30' to a timestamp using TO_TIMESTAMP?

easy

A. TO_TIMESTAMP('15/06/2024 14:30', 'DD/MM/YYYY HH24:MI')

B. TO_TIMESTAMP('15/06/2024 14:30', 'MM/DD/YYYY HH24:MI')

C. TO_TIMESTAMP('15/06/2024 14:30', 'YYYY-MM-DD HH24:MI')

D. TO_TIMESTAMP('15/06/2024 14:30', 'DD-MM-YYYY HH12:MI')

Solution

1. Step 1: Analyze the input string format

   The input is '15/06/2024 14:30' which is day/month/year with 24-hour time.

2. Step 2: Match format string correctly

   'DD/MM/YYYY HH24:MI' matches day/month/year and 24-hour hour:minute format.

3. Final Answer:

   TO_TIMESTAMP('15/06/2024 14:30', 'DD/MM/YYYY HH24:MI') -> Option A

4. Quick Check:

   Format must match input exactly [OK]

Hint: Match format tokens exactly to input string [OK]

Common Mistakes:

• Using MM for day or DD for month
• Using HH12 instead of HH24 for 24-hour time
• Wrong separator characters in format

3. What is the result of this query?

SELECT TO_TIMESTAMP('2024-06-15 08:45:30', 'YYYY-MM-DD HH24:MI:SS');

medium

A. String '2024-06-15 08:45:30'

B. 2024-06-15 (date only)

C. 2024-06-15 08:45:30 (timestamp without time zone)

D. Error: format mismatch

Solution

1. Step 1: Understand TO_TIMESTAMP output

   TO_TIMESTAMP converts text to a timestamp type including date and time.

2. Step 2: Check format matches input

   The format 'YYYY-MM-DD HH24:MI:SS' matches the input string exactly.

3. Final Answer:

   2024-06-15 08:45:30 (timestamp without time zone) -> Option C

4. Quick Check:

   TO_TIMESTAMP returns timestamp type [OK]

Hint: TO_TIMESTAMP includes time, TO_DATE does not [OK]

Common Mistakes:

• Expecting date only from TO_TIMESTAMP
• Using wrong format causing errors
• Confusing string output with timestamp type

4. Identify the error in this query:

SELECT TO_DATE('06-15-2024', 'DD-MM-YYYY');

medium

A. TO_DATE requires timestamp format, not date

B. TO_DATE cannot parse dates with dashes

C. Missing time part in format string

D. Format string does not match input date format

Solution

1. Step 1: Analyze input and format

   Input is '06-15-2024' which is month-day-year with dashes.

2. Step 2: Check format string correctness

   Format 'DD-MM-YYYY' expects day-month-year, which does not match input.

3. Final Answer:

   Format string does not match input date format -> Option D

4. Quick Check:

   Input and format must align exactly [OK]

Hint: Input and format must match order and separators [OK]

Common Mistakes:

• Swapping day and month in format
• Assuming TO_DATE accepts any format
• Ignoring separator characters

5. You have a text column event_time storing timestamps like '2024/06/15 07:30:00'. Which query correctly converts this to a timestamp and extracts the hour?

hard

A. SELECT EXTRACT(HOUR FROM TO_TIMESTAMP(event_time, 'YYYY/MM/DD HH24:MI:SS')) FROM events;

B. SELECT EXTRACT(HOUR FROM TO_DATE(event_time, 'YYYY/MM/DD HH24:MI:SS')) FROM events;

C. SELECT TO_TIMESTAMP(event_time, 'YYYY-MM-DD HH24:MI:SS') FROM events;

D. SELECT EXTRACT(HOUR FROM TO_TIMESTAMP(event_time, 'DD/MM/YYYY HH24:MI:SS')) FROM events;

Solution

1. Step 1: Choose correct function for timestamp with time

   TO_TIMESTAMP converts text with date and time to timestamp type; TO_DATE only returns date.

2. Step 2: Match format string to input

   Input uses slashes and format 'YYYY/MM/DD HH24:MI:SS' matches exactly.

3. Step 3: Extract hour from timestamp

   EXTRACT(HOUR FROM timestamp) returns the hour part correctly.

4. Final Answer:

   SELECT EXTRACT(HOUR FROM TO_TIMESTAMP(event_time, 'YYYY/MM/DD HH24:MI:SS')) FROM events; -> Option A

5. Quick Check:

   Use TO_TIMESTAMP with matching format to extract time parts [OK]

Hint: Use TO_TIMESTAMP for time data, TO_DATE for date only [OK]

Common Mistakes:

• Using TO_DATE when time is needed
• Mismatching format separators
• Wrong format order for date parts