Type casting with :: operator in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the :: operator do in PostgreSQL?

It converts a value from one data type to another, also called type casting.

beginner

How do you cast a string '123' to an integer using the :: operator?

You write '123'::integer to convert the string to an integer.

intermediate

Can you use the :: operator to cast between any two types?

No, only between compatible types. For example, casting text to integer works if the text is a valid number.

intermediate

What happens if you try to cast a non-numeric string to integer using ::?

PostgreSQL will return an error because the string cannot be converted to an integer.

intermediate

Is :: operator specific to PostgreSQL or standard SQL?

The :: operator is specific to PostgreSQL and some other databases; standard SQL uses the CAST() function.

What does '2024-06-01'::date do in PostgreSQL?

A Returns the string unchanged

B Converts the string to an integer

C Causes a syntax error

D Converts the string to a date type

The ::date casts the string to a date type.

Which of these is a valid use of the :: operator?

A <code>123::text</code>

B <code>'abc'::integer</code>

C <code>'true'::integer</code>

D <code>NULL::boolean</code>

Casting number 123 to text is valid. Casting 'abc' to integer or 'true' to integer is invalid. NULL can be cast to boolean and is valid.

What error occurs if you cast 'hello'::integer?

A No error, returns 0

B Syntax error

C Invalid input syntax for integer error

D Returns NULL

PostgreSQL throws an invalid input syntax error because 'hello' is not a number.

Which is the standard SQL alternative to :: for casting?

A CAST(value AS type)

B CONVERT(value, type)

C TO_TYPE(value)

D CHANGE_TYPE(value)

Standard SQL uses the CAST function to convert types.

What type will 123.45::integer return?

A 123.45 as float

B 123 as integer

C Error because of decimal

D NULL

Casting float to integer truncates the decimal part, returning 123.

Explain how the :: operator works in PostgreSQL and give an example.

Think about converting one data type to another.

What are the risks or errors you might face when using the :: operator?

Consider what happens if the value cannot be converted.

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