MySQL 简单查询提取 substring 给定 2 个分隔符

Question

Specifically I need to pick the part of field label from table tmp.label between delimiters <!-- rwbl_1 --> and <!-- rwbl_2 --> where label contains <span in order to be able to update that field for records erroneously formatted with HTML tags.

The result amounts to something like strip_tags . This is, obviously, only possible due to the presence of the abovementioned (or similar) delimiters.

Answer 1

Here is a simple solution for the abovementioned case to extract a substring given 2 delimiters.

SELECT
    SUBSTRING_INDEX(
        SUBSTRING_INDEX(label, 
            '<!-- rwbl_1 -->', -1), 
        '<!-- rwbl_2 -->', 1) AS cutout 
FROM tmp.label 
WHERE label LIKE '<span%' 
ORDER BY 1 
LIMIT 1;

More abstract:

SELECT
    SUBSTRING_INDEX(
        SUBSTRING_INDEX(string_field,        # field name
            'delimiter_1', -1),              # take the right part of original
        '<!-- delimiter_2 -->', 1)           # take the left part of the resulting substring
    AS cutout 
FROM my_table 
WHERE my_condition;

And the update now works like this:

UPDATE my_table 
SET string_field = SUBSTRING_INDEX(
        SUBSTRING_INDEX(string_field, 
            'delimiter_1', -1), 
        '<!-- delimiter_2 -->', 1) 
WHERE my_condition;