Scribbles for my bad memory

I met this issue when I SELECT CASE WHEN d.COUNTRY != 'AU' THEN 'Australia' ELSE countries.NAME END

BTW, I asked ChatGPT for help about this issue but unfortunately I didn't get a good answer.
One of the correct answers was

SELECT CASE WHEN d.COUNTRY != 'AU' THEN 'Australia' ELSE TO_CHAR(countries.NAME) END ...

I found the above answer myself and I told ChatGPT this answer. It replied

Great to hear that you were able to solve the issue! Yes, you can use the TO_CHAR function to convert the character set of the countries.NAME column to match the character set of your database. This is a common solution for character set mismatch errors. Using TO_CHAR function, you can explicitly specify the character set you want to convert to, like this:

TO_CHAR(countries.NAME, 'NLS_CHARACTERSET')

where 'NLS_CHARACTERSET' is the character set of your database. By default, Oracle uses the database's character set for character data that has no specified character set, so if you don't specify a character set for TO_CHAR, it will use the database's character set.

another answer is to use the CAST function in PL/SQL, here's an example from stackoverflow.com

Don't waste time; here's the sample code with the Oracle database.

SELECT * FROM SampleTable WHERE sample_field in (7,3,4,6) ORDER BY INSTR(',7,3,4,6,',  ',' || sample_field || ',');

In MySQL, we can

SELECT IF(A IS NULL, "something", A) FROM some_table;

If you want to do a similar thing in PL/SQL, you need to

SELECT (CASE WHEN A IS NULL THEN "something" ELSE A END) FROM some_table;

If you want to get the first non-null value from several fields, you have a better choice than using the CASE keyword.

SELECT COALESCE(A, B) FROM some_table

see more information about the Oracle COALESCE function

Today I first learnt the concept of CTE (Common Table Expression) from my colleague Rod. Basically, CTE can be used to improve the readability of your long and complex SQL statement.

for example, without CTE, you might write the following SQL:

CREATE OR REPLACE VIEW dfx.test_vw123 AS
SELECT 
	a.FIELD_A, 
	a.FIELD_B, 
	b.FIELD_C
	b.FIELD_D
FROM (
	SELECT 
		MAX(FIELD_A) AS FIELD_A, 
		COUNT(*) AS FIELD_B 
	FROM TABLE_A ta 
	WHERE ta.FIELD_E = 'General' 
	GROUP BY ta.FIELD_F
) a 
LEFT JOIN TABLE_A b 
ON a.FIELD_A = b.FIELD_A

with CTE, you could write the following one with better readability.

CREATE OR REPLACE VIEW dfx.test_vw123 AS
WITH TABLE_A_STATS AS (
	SELECT 
		MAX(FIELD_A) AS FIELD_A, 
		COUNT(*) AS FIELD_B 
	FROM TABLE_A ta 
	WHERE ta.FIELD_E = 'General' 
	GROUP BY ta.FIELD_F
)
SELECT 
	a.FIELD_A, 
	a.FIELD_B, 
	b.FIELD_C
	b.FIELD_D
FROM TABLE_A_STATS a 
LEFT JOIN TABLE_A b 
ON a.FIELD_A = b.FIELD_A;

For further information about CTE, click Reference
I also found another article about CTE in Chinese.