如何编写这样的MySQL语句？

Question

create table test(
  id ...
  weight ..
);

How to write such a statement so that the weight of each record is at least the weight of previous record plus 1?

recordN.weight >=(recordN-1).weight+1

Answer 1

you need to check it before inserting in to table, eg you can get the last row from the table with query

select weight from test where id = (select MAX(id) from test);

then compare this weight value from the values you are inserting and

if (newWeight >= weight)
{
   //insert here
}

Answer 2

One of the ways is to store the weight in a variable and then use it in an update statement

SELECT weight FROM test ORDER BY id DESC LIMIT 1 INTO @weight;
INSERT INTO test (weight) VALUES (@weight+1);

That will get the weight of the previous record and use it in the next insert statement (assuming you have auto increment ids).

You can also concatenate the query:

INSERT INTO test (weight) SELECT (weight+1) FROM test ORDER BY id DESC LIMIT 1;

Answer 3

Sounds strange. Are you updating existing data, adding a new row to the table or selecting the data sorted by weight?

Sorting is easiest, use SELECT ... ORDER BY weight;

Adding a row, you could use the previous answer: INSERT INTO test (weight) (SELECT (max(weight)+1) FROM test);

Although note that (1) There is an autoincrement function in MySQL if that's what you're looking for; (2) in good DB tables, row order is immaterial; rows can be sorted without information being lost. If you are relying on row order for anything, consider your database design again.

Finally, if you're trying to update the data, adding to the weight, it's a common problem. I always have to look it up online, so I'll leave you to do that :)