MySQL查询像数字大于x

Question

I have a field for comments used to store the title of the item sold on the site as well as the bid number (bid_id). Unfortunately, the bid_id is not stored on its own in that table.

I want to query items that have a number (the bid_id) greater than 4,000 for example.

So, what I have is:

select * from mysql_table_name where comment like '< 4000'

I know this won't work, but I need something similar that works.

Thanks a lot!

Answer 1

There is nothing ready that works like that.

You could write a custom function or loadable UDF , but it would be a significant work, with significant impact on the database. Then you could run WHERE GET_BID_ID(comment) < 4000 .

What you can do more easily is devise some way of extracting the bid_id using available string functions .

For example if the bid_id is always in the last ten characters, you can extract those, and replace all characters that are not digits with nil. What is left is the bid_id , and that you can compare.

Of course you need a complex expression with LENGTH() , SUBSTRING() , and REPLACE() . If the bid_id is between easily recognizable delimiters, then perhaps SUBSTRING_INDEX() is more your friend.

But better still... add an INTEGER column, initialize it to null, then store there the extracted bid_id . Or zero if you're positive there's no bid_id . Having data stored in mixed contexts is evil (and a known SQL antipattern to boot). Once you have the column available, you can select every few seconds a small number of items with new_bid_id still NULL and subject those to extraction, thereby gradually amending the database without overloading the system.

In practice

This is the same approach one would use with more complicated cases. We start by checking what we have (this is a test table)

SELECT commento FROM arti LIMIT 3;

+-----------------------------------------+
| commento                                |
+-----------------------------------------+
| This is the first comment 100 200 42500 |
| Another 7 Q 32768                       |
| And yet another 200 15 55332            |
+-----------------------------------------+

So we need the last characters:

SELECT SUBSTRING(commento, LENGTH(commento)-5) FROM arti LIMIT 3;
+-----------------------------------------+
| SUBSTRING(commento, LENGTH(commento)-5) |
+-----------------------------------------+
|  42500                                  |
|  32768                                  |
|  55332                                  |
+-----------------------------------------+

This looks good but it is not; there's an extra space left before the ID. So 5 doesn't work, SUBSTRING is 1-based. No matter; we just use 4.

...and we're done.

mysql> SELECT commento FROM arti WHERE SUBSTRING(commento, LENGTH(commento)-4) < 40000;
+-------------------+
| commento          |
+-------------------+
| Another 7 Q 32768 |
+-------------------+

mysql> SELECT commento FROM arti WHERE SUBSTRING(commento, LENGTH(commento)-4) BETWEEN 35000 AND 55000;
+-----------------------------------------+
| commento                                |
+-----------------------------------------+
| This is the first comment 100 200 42500 |
+-----------------------------------------+

The problem is if you have a number not of the same length (eg 300 and 131072). Then you need to take a slice large enough for the larger number, and if the number is short, you will get maybe "1 5 300" in your slice. That's where SUBSTRING_INDEX comes to the rescue: by capturing seven characters, from " 131072" to "1 5 300", the ID will always be in the last space separated token of the slice .

IN THIS LAST CASE , when numbers are not of the same length, you will find a problem. The extracted IDs are not numbers at all - to MySQL, they are strings . Which means that they are compared in lexicographic, not numerical, order; and "17534" is considered smaller than "202", just like "Alice" comes before "Bob". To overcome this you need to cast the string as unsigned integer, which further slows down the operations.

WHERE CAST( SUBSTRING(...) AS UNSIGNED) < 4000

Answer 2

select * from mysql_table_name where substring(comment,start,length, signed integer) < 4000

This will work, but I suggest create new column and put the bid value in it then compare.

To update value in new column you can use

update table set newcol = substring(comment,start,length)

Hope this will help

Answer 3

Just get your bid_id column cleaned up. Then index is.

create table `prior`
(   id int auto_increment primary key,
    comments text not null
);
insert `prior` (comments) values ('asdfasdf adfas d d 93827363'),('mouse cat 12345678');
alter table `prior` add column bid_id int; -- add a nullable int column
select * from `prior`; -- bid_id is null atm btw
update `prior` set bid_id=right(comments,8); -- this will auto-cast to an int
select * from `prior`; 
+----+-----------------------------+----------+
| id | comments                    | bid_id   |
+----+-----------------------------+----------+
|  1 | asdfasdf adfas d d 93827363 | 93827363 |
|  2 | mouse cat 12345678          | 12345678 |
+----+-----------------------------+----------+

Create the index:

CREATE INDEX `idxBidId` ON `prior` (bid_id); -- or unique index