Set the maximum allowed number of connections in MySQL

Answer:

MySQL has a setting which prevent the database from overloaded due to too many concurrent connections. This value should depends on the processing power of your database server, e.g. CPU, memory, disks I/O.

To change the value, edit the MySQL configurations, e.g. /etc/my.cnf

..
max_connections=100
..

So if a client connecting to the database exceeded this value, they will be blocked.

Don't forget to restart MySQL to take effect.

# /sbin/service mysqld restart

Sleep in MySQL

Answer:

In MySQL, you can issue an query to sleep(), e.g.

mysql> SELECT SLEEP(2);
+----------+
| SLEEP(2) |
+----------+
|        0 |
+----------+
1 row in set (2.00 sec)

Compare binary value in MySQL

Answer:

Assume you have the following table.

mysql> CREATE TABLE t ( 
    c BINARY(3)
) ENGINE = INNODB;

And you insert some sample value...

mysql> INSERT INTO t VALUES ('fo');
mysql> INSERT INTO t VALUES ('foo');

Finally you select it

mysql> SELECT * FROM t WHERE c = 'foo';
+------+
| c    |
+------+
| foo  |
+------+
1 row in set (0.00 sec)

mysql> SELECT * FROM t WHERE c = 'fo';
Empty set (0.00 sec)

Why you cannot find the result in the second query? Because the column is BINARY, the trailing space(s) is not ignored like this.

You need to query like...

mysql> SELECT * FROM WHERE c = 'fo\0';
+------+
| c    |
+------+
| fo   |
+------+
1 row in set (0.00 sec)

As you can see, you need to append the the 0x00 (the zero byte). at the end of the character.