Type casting with :: operator in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to cast the string '123' to an integer using the :: operator.

PostgreSQL

SELECT '123'[1]integer;

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

A->

B=>

C::

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using -> or => instead of :: for casting.

Trying to use == which is a comparison operator.

The :: operator in PostgreSQL is used to cast a value to a specific type. Here, '123'::integer converts the string to an integer.

2fill in blank

medium

Complete the code to cast the integer 45 to text using the :: operator.

PostgreSQL

SELECT 45[1]text;

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

A->

B::

C=>

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using -> or => which are not casting operators.

Using == which is for comparison, not casting.

Using 45::text converts the integer 45 to a text string in PostgreSQL.

3fill in blank

hard

Fix the error in the code to correctly cast the string '3.14' to a numeric type.

PostgreSQL

SELECT '3.14'[1]numeric;

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

A->

B=>

C==

D::

Attempts:

3 left

💡 Hint

Common Mistakes

Using -> or => which cause syntax errors.

Using == which is a comparison operator.

The correct way to cast a string to numeric in PostgreSQL is using the :: operator: '3.14'::numeric.

4fill in blank

hard

Fill both blanks to cast the string '2024-06-01' to a date and then to text.

PostgreSQL

SELECT ('2024-06-01'[1]date)[2]text;

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

A::

B->

D=>

Attempts:

3 left

💡 Hint

Common Mistakes

Using -> or => instead of :: for casting.

Forgetting parentheses around the first cast.

First, cast the string to date using ::date, then cast the date to text using ::text.

5fill in blank

hard

Fill all three blanks to cast the string '100' to integer, multiply by 2, and cast the result to text.

PostgreSQL

SELECT (( '100'[1]integer ) * [2] )[3]text;

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

A::

B2

D->

Attempts:

3 left

💡 Hint

Common Mistakes

Using -> instead of :: for casting.

Forgetting to multiply by 2.

Not casting the final result to text.

First cast '100' to integer with ::integer, multiply by 2, then cast the result to text with ::text.

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