Type casting with :: operator in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this type casting query?

Consider the following PostgreSQL query:

SELECT '123'::integer + 10 AS result;

What is the value of result?

PostgreSQL

SELECT '123'::integer + 10 AS result;

A '12310'

B 133

C Syntax error

D Type error

💡 Hint

Casting '123' to integer allows arithmetic addition.

The string '123' is cast to integer 123, then 10 is added, resulting in 133.

📝 Syntax

intermediate

Which option causes an error in type casting?

Which of the following PostgreSQL queries will cause an error due to invalid use of the :: operator?

A SELECT 'abc'::integer;

B SELECT 123::text;

C SELECT '123'::integer + 10;

D SELECT '2023-01-01'::date;

💡 Hint

Casting a non-numeric string to integer is invalid.

Option A tries to cast 'abc' to integer, which causes a runtime error (invalid input syntax for type integer: "abc"). The other options are valid.

❓ query_result

advanced

What is the output of this complex type casting?

Given the query:

SELECT ('2023-06-15 14:30:00'::timestamp)::date AS cast_date;

What is the value of cast_date?

PostgreSQL

SELECT ('2023-06-15 14:30:00'::timestamp)::date AS cast_date;

A Type error

B 2023-06-15 14:30:00

C Syntax error

D 2023-06-15

💡 Hint

Casting timestamp to date removes the time part.

The timestamp is first cast from string, then cast to date, which keeps only the date part.

🔧 Debug

advanced

Why does this query fail?

Examine the query:

SELECT '12.34'::integer;

Why does this query fail?

PostgreSQL

SELECT '12.34'::integer;

A Because integer type does not exist in PostgreSQL

B Because the :: operator is not valid for casting strings

C Because '12.34' is a string that cannot be cast to integer due to decimal point

D Because the query is missing a FROM clause

💡 Hint

Integer casting requires whole numbers without decimals.

The string '12.34' contains a decimal point, so casting to integer fails with invalid input syntax error.

❓ optimization

expert

Which query is more efficient for casting and filtering?

You want to select rows where the text column price_text represents an integer greater than 100.

Which query is more efficient in PostgreSQL?

A SELECT * FROM products WHERE price_text::integer > 100;

B SELECT * FROM products WHERE price_text > '100';

C SELECT * FROM products WHERE CAST(price_text AS integer) > 100;

D SELECT * FROM products WHERE price_text::text > '100';

💡 Hint

The :: operator is a shorthand for CAST and often preferred for readability and performance.

Option A uses :: operator which is concise and efficient. Option A is equivalent but more verbose. Options C and D compare strings lexicographically, which is incorrect for numeric comparison.

Practice

1. What does the PostgreSQL expression '123'::integer do?

easy

A. Causes a syntax error because :: is invalid

B. Converts the integer 123 to text '123'

C. Converts the text '123' to an integer number 123

D. Leaves the value as text without any change

Solution

1. Step 1: Understand the :: operator

   The :: operator in PostgreSQL is used to cast or convert a value from one data type to another.

2. Step 2: Apply casting to the given expression

   The expression '123'::integer converts the string '123' into the integer 123.

3. Final Answer:

   Converts the text '123' to an integer number 123 -> Option C

4. Quick Check:

   Type casting '123'::integer = 123 [OK]

Hint: Remember :: changes type, here text to integer [OK]

Common Mistakes:

• Thinking :: converts integer to text
• Assuming :: is invalid syntax
• Believing the value stays as text

2. Which of the following is the correct syntax to cast a column named price to numeric in PostgreSQL?

easy

A. SELECT price AS numeric FROM products;

B. SELECT price to numeric FROM products;

C. SELECT CAST(price numeric) FROM products;

D. SELECT price::numeric FROM products;

Solution

1. Step 1: Recall correct casting syntax

   In PostgreSQL, the correct way to cast a value is using value::type or CAST(value AS type).

2. Step 2: Identify the correct option

   SELECT price::numeric FROM products; uses price::numeric, which is valid syntax. Options B, C, and D have incorrect syntax.

3. Final Answer:

   SELECT price::numeric FROM products; -> Option D

4. Quick Check:

   Correct cast syntax uses :: [OK]

Hint: Use ::type after value for casting [OK]

Common Mistakes:

• Using 'to' instead of :: or CAST
• Incorrect CAST syntax missing AS
• Using AS for casting instead of aliasing

3. What is the output of this query?

SELECT '2023-06-01'::date + 5;

medium

A. '2023-06-06' (date 5 days after June 1, 2023)

B. 2023-06-01 05:00:00 (timestamp with 5 hours added)

C. Error: cannot add integer to date

D. '2023-06-01 5' (string concatenation)

Solution

1. Step 1: Cast string to date

   The expression '2023-06-01'::date converts the text to a date value representing June 1, 2023.

2. Step 2: Add integer to date

   Adding 5 to a date adds 5 days, resulting in June 6, 2023.

3. Final Answer:

   '2023-06-06' (date 5 days after June 1, 2023) -> Option A

4. Quick Check:

   Date + integer adds days [OK]

Hint: Adding integer to date adds days [OK]

Common Mistakes:

• Thinking integer adds hours or seconds
• Expecting string concatenation
• Assuming error on adding integer to date

4. Identify the error in this query:

SELECT 'abc'::integer;

medium

A. Runtime error: invalid input syntax for integer

B. Syntax error due to invalid cast operator

C. Returns NULL because 'abc' is not a number

D. No error, returns 0

Solution

1. Step 1: Understand casting invalid text to integer

   Trying to cast a non-numeric string like 'abc' to integer causes a runtime error in PostgreSQL.

2. Step 2: Identify error type

   The error message is 'invalid input syntax for integer', indicating the cast failed at runtime.

3. Final Answer:

   Runtime error: invalid input syntax for integer -> Option A

4. Quick Check:

   Invalid cast causes runtime error [OK]

Hint: Casting non-numeric text to integer causes error [OK]

Common Mistakes:

• Expecting NULL instead of error
• Thinking syntax error instead of runtime error
• Assuming automatic conversion to 0

5. You have a table orders with a text column order_date storing dates as strings like '06/01/2023'. You want to find all orders after June 1, 2023. Which query correctly uses type casting with :: to achieve this?

hard

A. SELECT * FROM orders WHERE order_date > '2023-06-01'::date;

B. SELECT * FROM orders WHERE order_date::date > '2023-06-01';

C. SELECT * FROM orders WHERE order_date::text > '2023-06-01'::text;

D. SELECT * FROM orders WHERE order_date::integer > 20230601;

Solution

1. Step 1: Understand data types involved

   The column order_date is text, so to compare dates properly, it must be cast to date type.

2. Step 2: Choose correct casting and comparison

   SELECT * FROM orders WHERE order_date::date > '2023-06-01'; casts order_date to date and compares it to a date literal, which is correct. SELECT * FROM orders WHERE order_date > '2023-06-01'::date; compares text to date, which is invalid. SELECT * FROM orders WHERE order_date::text > '2023-06-01'::text; compares text to text, which is lexicographical, not date. SELECT * FROM orders WHERE order_date::integer > 20230601; tries to cast text to integer, which will cause error.

3. Final Answer:

   SELECT * FROM orders WHERE order_date::date > '2023-06-01'; -> Option B

4. Quick Check:

   Cast text column to date before date comparison [OK]

Hint: Cast text column to date before comparing dates [OK]

Common Mistakes:

• Comparing text directly to date
• Casting to wrong type like integer
• Comparing text lexicographically instead of date