MySQL限制

Question

I am trying to understand the following snippet of a complicated MYSQL procedure, where var1, var2, var3 and var4 are declared variables:

 select col1, col2, col3, col4 from table order by col1 limit 5, 1 into var1, var2, var3, var4; 

In particular, I don't know what the above is inserting into var1, var2, var3 and var4.

More generally, I don't understand what the following does in MYSQL:

 select...limit ..., 1 into 

Any help would be great!

Answer 1

Lets dissect the query.

select col1, col2, col3, col4 from table;

This gives you all the rows from the table, but only with 4 columns.

select col1, col2, col3, col4 from table order by col1;

Now you are are ordering the results in the ascending order of col1.

select col1, col2, col3, col4 from table order by col1 limit 5, 1;

Here 5 is the offset and 1 is the actual limit , which will reduce your result set to a size of 1.

What does offset mean?

Suppose you get 10 results from the second query. Then this third query will give you results starting from the 6th row and limited by the limit passed, which in this case is 1. Hence, you will only get one row having four values.

What does into do?

It simply stores the values of those 4 results into those 4 variables.

Answer 2

So LIMIT can take two paramaters LIMIT offset, count

offset is the offset of the first row you want to return (for example offset of row 1 would be 0)

count is the max amount of rows that you want to return

If you pass it just one parameter then that's just a count so LIMIT 5 would return the first 5 rows in the table.

so your query is returning the 4 values of col1, col2, col3 and col4 that are in row 6 and then is returning those values into var1, var2, var3, var4

(I think... I haven't used anything like that in a while... so I stand to be corrected)