Case Syntax Errors

问题:

Hello I am attempting to do this with mySQL. I tried using SELECT IF and CASE to alter titles. Every title description has "A " in front of the description; even if the second word starts with a consonant. So I'm trying to query the descriptions but edit the ones with a leading consonant to start with "An ".

CASE
    WHEN description LIKE ('A A%') THEN CONCAT('An ', TRIM(LEADING 'A ' FROM description))
    WHEN description LIKE ('A E%') THEN CONCAT('An ', TRIM(LEADING 'A ' FROM description))
    WHEN description LIKE ('A I%') THEN CONCAT('An ', TRIM(LEADING 'A ' FROM description))
    WHEN description LIKE ('A O%') THEN CONCAT('An ', TRIM(LEADING 'A ' FROM description))
    WHEN description LIKE ('A U%') THEN CONCAT('An ', TRIM(LEADING 'A ' FROM description))
END;

So this particular code was just giving me Error Code: 1064 syntax.

SELECT IF(description NOT LIKE ('A A%'), NULL, CONCAT('An ', TRIM(LEADING 'A ' FROM description))),
    IF(description NOT LIKE ('A E%'), null, CONCAT('An ', TRIM(LEADING 'A ' FROM description))),
    IF(description NOT LIKE ('A I%'), NULL, CONCAT('An ', TRIM(LEADING 'A ' FROM description))),
    IF(description NOT LIKE ('A O%'), NULL, CONCAT('An ', TRIM(LEADING 'A ' FROM description))),
    IF(description NOT LIKE ('A U%'), NULL, CONCAT('An ', TRIM(LEADING 'A ' FROM description)))
FROM film_text;    

This particular block of code returns separate columns however I'm trying to query all of this into one column.

What my IF block returns:

回答1:

Don't use parentheses around the terms in your LIKE expressions. This is probably the cause of the syntax error you are seeing. Instead, try using this CASE expression having just a single condition:

SELECT CASE WHEN UPPER(description) NOT LIKE 'A A%' AND
                 UPPER(description) NOT LIKE 'A E%' AND
                 UPPER(description) NOT LIKE 'A I%' AND
                 UPPER(description) NOT LIKE 'A O%' AND
                 UPPER(description) NOT LIKE 'A U%'
            THEN CONCAT('An ', TRIM(LEADING 'A ' FROM description))   -- replace A with An
            ELSE description      -- otherwise use original description
       END AS description
FROM yourTable

You could also use REGEXP here with the negative character class [^aeiou] to match titles not beginning with a vowel:

SELECT CASE WHEN description REGEXP '^A [^aeiou]'
            THEN CONCAT('An ', TRIM(LEADING 'A ' FROM description))
            ELSE description
       END AS description
FROM yourTable

回答2:

Your question is really several questions.

To get data from multiple "columns" returned as a single column, you'd use separate queries and use UNION, between them.

But this isn't really your issue.

It might be better to do the check (the "if") in the WHERE clause, and the replacement only once in SELECT.

This is untested, but may get you closer to what you want:

SELECT 
  CONCAT('An ', TRIM(LEADING 'A ' FROM description)) AS descrip
FROM film_text
WHERE description REGEXP '^A [aeiou]';    

The REGEXP does the check you were trying to perform in your CASE, and limits the result set to those rows only.

To do the whole thing, and return all descriptions, you can combine the REGEX syntax with the CASE you initially tried.

SELECT 
  CASE WHEN description REGEXP '^A [aeiou]'
    THEN CONCAT('An ', TRIM(LEADING 'A ' FROM description))
    ELSE description
  END AS description
FROM film_text;