3.21.1 Problem
LIMIT usually works best in conjunction with an ORDER BY clause that sorts rows. But sometimes the sort order is the opposite of what you want for the final result.
3.21.2 Solution
Rewrite the query, or write a program that retrieves the rows and sorts them into the order you want.
3.21.3 Discussion
If you want the last four records of a result set, you can obtain them easily by sorting the set in reverse order and using LIMIT 4. For example, the following query returns the names and birth dates for the four people in the profile table who were born most recently:
mysql> SELECT name, birth FROM profile ORDER BY birth DESC LIMIT 4; +---------+------------+ | name | birth | +---------+------------+ | Shepard | 1975-09-02 | | Carl | 1973-11-02 | | Fred | 1970-04-13 | | Mort | 1969-09-30 | +---------+------------+
But that requires sorting the birth values in descending order to place them at the head of the result set. What if you want them in ascending order instead? One way to solve this problem is to use two queries. First, use COUNT( ) to find out how many rows are in the table:
mysql> SELECT COUNT(*) FROM profile; +----------+ | COUNT(*) | +----------+ | 10 | +----------+
Then, sort the values in ascending order and use the two-argument form of LIMIT to skip all but the last four records:
mysql> SELECT name, birth FROM profile ORDER BY birth LIMIT 6, 4; +---------+------------+ | name | birth | +---------+------------+ | Mort | 1969-09-30 | | Fred | 1970-04-13 | | Carl | 1973-11-02 | | Shepard | 1975-09-02 | +---------+------------+
Single-query solutions to this problem may be available if you're issuing queries from within a program and can manipulate the query result. For example, if you fetch the values into a data structure, you can reverse the order of the values in the structure. Here is some Perl code that demonstrates this approach:
my $stmt = "SELECT name, birth FROM profile ORDER BY birth DESC LIMIT 4"; # fetch values into a data structure my $ref = $dbh->selectall_arrayref ($stmt); # reverse the order of the items in the structure my @val = reverse (@{$ref}); # use $val[$i] to get a reference to row $i, then use # $val[$i]->[0] and $val[$i]->[1] to access column values
Alternatively, you can simply iterate through the structure in reverse order:
my $stmt = "SELECT name, birth FROM profile ORDER BY birth DESC LIMIT 4"; # fetch values into a data structure my $ref = $dbh->selectall_arrayref ($stmt); # iterate through the structure in reverse order my $row_count = @{$ref}; for (my $i = $row_count - 1; $i >= 0; $i--) { # use $ref->[$i]->[0] and $ref->[$i]->[1] here... }
Using the mysql Client Program
Writing MySQL-Based Programs
Record Selection Techniques
Working with Strings
Working with Dates and Times
Sorting Query Results
Generating Summaries
Modifying Tables with ALTER TABLE
Obtaining and Using Metadata
Importing and Exporting Data
Generating and Using Sequences
Using Multiple Tables
Statistical Techniques
Handling Duplicates
Performing Transactions
Introduction to MySQL on the Web
Incorporating Query Resultsinto Web Pages
Processing Web Input with MySQL
Using MySQL-Based Web Session Management
Appendix A. Obtaining MySQL Software
Appendix B. JSP and Tomcat Primer
Appendix C. References