根据mysql手册，mysql的变量分为两种：系统变量和用户变量。但是在实际使用中，还会遇到诸如局部变量、会话变量等概念。根据个人感觉，mysql变量大体可以分为四种类型：
一、局部变量。
局部变量一般用在sql语句块中，比如存储过程的begin/end。其作用域仅限于该语句块，在该语句块执行完毕后，局部变量就消失了。
局部变量一般用declare来声明，可以使用default来说明默认值。
例如在存储过程中定义局部变量：
drop procedure if exists add;
create procedure add
(
in a int,
in b int
)
begin
declare c int default 0;
set c = a + b;
select c as c;
end;
在上述存储过程中定义的变量c就是局部变量
二、用户变量。
用户变量的作用域要比局部变量要广。用户变量可以作用于当前整个连接，但是当当前连接断开后，其所定义的用户变量都会消失。
用户变量使用如下(这里我们无须使用declare关键字进行定义，可以直接这样使用):
select @变量名
对用户变量赋值有两种方式，一种是直接用"="号，另一种是用":="号。其区别在于使用set命令对用户变量进行赋值时，两种方式都可以使用；当使用select语句对用户变量进行赋值时，只能使用":="方式，因为在select语句中，"="号被看作是比较操作符。
示例程序如下：
drop procedure if exists math;
create procedure math
(
in a int,
in b int
)
begin
set @var1 = 1;
set @var2 = 2;
select @sum:=(a + b) as sum, @dif:=(a - b) as dif;
end;
mysql> call math(3, 4);
+------+------+| sum | dif |+------+------+| 7 | -1 | +------+------+1 row in set (0.00 sec)Query OK, 0 rows affected (0.00 sec)mysql> select @var1; //var1为用户变量+-------+| @var1 |+-------+| 1 | +-------+1 row in set (0.00 sec)mysql> select @var2; //var2为用户变量+-------+| @var2 |+-------+| 2 | +-------+1 row in set (0.00 sec)
3、会话变量
服务器为每个连接的客户端维护一系列会话变量。在客户端连接时，使用相应全局变量的当前值对客户端的会话变量进行初始化。设置会话变量不需要特殊权限，但客户端只能更改自己的会话变量，而不能更改其它客户端的会话变量。会话变量的作用域与用户变量一样，仅限于当前连接。当当前连接断开后，其设置的所有会话变量均失效。
设置会话变量有如下三种方式：
set session var_name = value;
set @@session.var_name = value;
set var_name = value;
查看一个会话变量也有如下三种方式：
select @@var_name;
select @@session.var_name;
show session variables like "%var%";
mysql> show session variables;
+---------------------------------+------------------------------------------------------------+| Variable_name | Value |+---------------------------------+------------------------------------------------------------+| auto_increment_increment | 1 || auto_increment_offset | 1 || automatic_sp_privileges | ON || back_log | 50 || basedir | /usr/ || bdb_cache_size | 8388600 || bdb_home | /var/lib/mysql/ || bdb_log_buffer_size | 32768 || bdb_logdir | || bdb_max_lock | 10000 || bdb_shared_data | OFF || bdb_tmpdir | /tmp/ || binlog_cache_size | 32768 || bulk_insert_buffer_size | 8388608 || character_set_client | latin1 || character_set_connection | latin1 || character_set_database | latin1 || character_set_filesystem | binary || character_set_results | latin1 || character_set_server | latin1 || character_set_system | utf8 || character_sets_dir | /usr/share/mysql/charsets/ || collation_connection | latin1_swedish_ci || collation_database | latin1_swedish_ci || collation_server | latin1_swedish_ci || completion_type | 0 || concurrent_insert | 1 || connect_timeout | 5 || datadir | /var/lib/mysql/ || date_format | %Y-%m-%d || datetime_format | %Y-%m-%d %H:%i:%s || default_week_format | 0 || delay_key_write | ON || delayed_insert_limit | 100 || delayed_insert_timeout | 300 || delayed_queue_size | 1000 || div_precision_increment | 4 || keep_files_on_create | OFF || engine_condition_pushdown | OFF || expire_logs_days | 0 || flush | OFF || flush_time | 0 || ft_boolean_syntax | + -><()~*:""&| || ft_max_word_len | 84 || ft_min_word_len | 4 || ft_query_expansion_limit | 20 || ft_stopword_file | (built-in) || group_concat_max_len | 1024 || have_archive | NO || have_bdb | YES || have_blackhole_engine | NO || have_compress | YES || have_crypt | YES || have_csv | NO || have_dynamic_loading | YES || have_example_engine | NO || have_federated_engine | NO || have_geometry | YES || have_innodb | YES || have_isam | NO || have_merge_engine | YES || have_ndbcluster | NO || have_openssl | DISABLED || have_ssl | DISABLED || have_query_cache | YES || have_raid | NO || have_rtree_keys | YES || have_symlink | YES || hostname | localhost || init_connect | || init_file | || init_slave | || innodb_additional_mem_pool_size | 1048576 || innodb_autoextend_increment | 8 || innodb_buffer_pool_awe_mem_mb | 0 || innodb_buffer_pool_size | 8388608 || innodb_checksums | ON || innodb_commit_concurrency | 0 || innodb_concurrency_tickets | 500 || innodb_data_file_path | ibdata1:10M:autoextend || innodb_data_home_dir | || innodb_doublewrite | ON || innodb_fast_shutdown | 1 || innodb_file_io_threads | 4 || innodb_file_per_table | OFF || innodb_flush_log_at_trx_commit | 1 || innodb_flush_method | || innodb_force_recovery | 0 || innodb_lock_wait_timeout | 50 || innodb_locks_unsafe_for_binlog | OFF || innodb_log_arch_dir | || innodb_log_archive | OFF || innodb_log_buffer_size | 1048576 || innodb_log_file_size | 5242880 || innodb_log_files_in_group | 2 || innodb_log_group_home_dir | ./ || innodb_max_dirty_pages_pct | 90 || innodb_max_purge_lag | 0 || innodb_mirrored_log_groups | 1 || innodb_open_files | 300 || innodb_rollback_on_timeout | OFF || innodb_support_xa | ON || innodb_sync_spin_loops | 20 || innodb_table_locks | ON || innodb_thread_concurrency | 8 || innodb_thread_sleep_delay | 10000 || interactive_timeout | 28800 || join_buffer_size | 131072 || key_buffer_size | 8388600 || key_cache_age_threshold | 300 || key_cache_block_size | 1024 || key_cache_division_limit | 100 || language | /usr/share/mysql/english/ || large_files_support | ON || large_page_size | 0 || large_pages | OFF || lc_time_names | en_US || license | GPL || local_infile | ON || locked_in_memory | OFF || log | OFF || log_bin | OFF || log_bin_trust_function_creators | OFF || log_error | || log_queries_not_using_indexes | OFF || log_slave_updates | OFF || log_slow_queries | OFF || log_warnings | 1 || long_query_time | 10 || low_priority_updates | OFF || lower_case_file_system | OFF || lower_case_table_names | 0 || max_allowed_packet | 1048576 || max_binlog_cache_size | 4294967295 || max_binlog_size | 1073741824 || max_connect_errors | 10 || max_connections | 100 || max_delayed_threads | 20 || max_error_count | 64 || max_heap_table_size | 16777216 || max_insert_delayed_threads | 20 || max_join_size | 4294967295 || max_length_for_sort_data | 1024 || max_prepared_stmt_count | 16382 || max_relay_log_size | 0 || max_seeks_for_key | 4294967295 || max_sort_length | 1024 || max_sp_recursion_depth | 0 || max_tmp_tables | 32 || max_user_connections | 0 || max_write_lock_count | 4294967295 || multi_range_count | 256 || myisam_data_pointer_size | 6 || myisam_max_sort_file_size | 2147483647 || myisam_recover_options | OFF || myisam_repair_threads | 1 || myisam_sort_buffer_size | 8388608 || myisam_stats_method | nulls_unequal || net_buffer_length | 16384 || net_read_timeout | 30 || net_retry_count | 10 || net_write_timeout | 60 || new | OFF || old_passwords | ON || open_files_limit | 1024 || optimizer_prune_level | 1 || optimizer_search_depth | 62 || pid_file | /var/run/mysqld/mysqld.pid || port | 3306 || preload_buffer_size | 32768 || profiling | OFF || profiling_history_size | 15 || protocol_version | 10 || query_alloc_block_size | 8192 || query_cache_limit | 1048576 || query_cache_min_res_unit | 4096 || query_cache_size | 0 || query_cache_type | ON || query_cache_wlock_invalidate | OFF || query_prealloc_size | 8192 || range_alloc_block_size | 2048 || read_buffer_size | 131072 || read_only | OFF || read_rnd_buffer_size | 262144 || relay_log_purge | ON || relay_log_space_limit | 0 || rpl_recovery_rank | 0 || secure_auth | OFF || secure_file_priv | || server_id | 0 || skip_external_locking | ON || skip_networking | OFF || skip_show_database | OFF || slave_compressed_protocol | OFF || slave_load_tmpdir | /tmp/ || slave_net_timeout | 3600 || slave_skip_errors | OFF || slave_transaction_retries | 10 || slow_launch_time | 2 || socket | /var/lib/mysql/mysql.sock || sort_buffer_size | 2097144 || sql_big_selects | ON || sql_mode | || sql_notes | ON || sql_warnings | OFF || ssl_ca | || ssl_capath | || ssl_cert | || ssl_cipher | || ssl_key | || storage_engine | MyISAM || sync_binlog | 0 || sync_frm | ON || system_time_zone | CST || table_cache | 64 || table_lock_wait_timeout | 50 || table_type | MyISAM || thread_cache_size | 0 || thread_stack | 196608 || time_format | %H:%i:%s || time_zone | SYSTEM || timed_mutexes | OFF || tmp_table_size | 33554432 || tmpdir | /tmp/ || transaction_alloc_block_size | 8192 || transaction_prealloc_size | 4096 || tx_isolation | REPEATABLE-READ || updatable_views_with_limit | YES || version | 5.0.51a || version_bdb | Sleepycat Software: Berkeley DB 4.1.24: (January 11, 2008) || version_comment | Source distribution || version_compile_machine | i386 || version_compile_os | redhat-linux-gnu || wait_timeout | 28800 |+---------------------------------+------------------------------------------------------------+234 rows in set (0.00 sec)
4、全局变量
全局变量影响服务器整体操作。当服务器启动时，它将所有全局变量初始化为默认值。这些默认值可以在选项文件中或在命令行中指定的选项进行更改。要想更改全局变量，必须具有SUPER权限。全局变量作用于server的整个生命周期，但是不能跨重启。即重启后所有设置的全局变量均失效。要想让全局变量重启后继续生效，需要更改相应的配置文件。
要设置一个全局变量，有如下两种方式：
set global var_name = value; //注意：此处的global不能省略。根据手册，set命令设置变量时若不指定GLOBAL、SESSION或者LOCAL，默认使用SESSION
set @@global.var_name = value; //同上
要想查看一个全局变量，有如下两种方式：
select @@global.var_name;
show global variables like "%var%";
mysql> show global variables;
+---------------------------------+------------------------------------------------------------+| Variable_name | Value |+---------------------------------+------------------------------------------------------------+| auto_increment_increment | 1 || auto_increment_offset | 1 || automatic_sp_privileges | ON || back_log | 50 || basedir | /usr/ || bdb_cache_size | 8388600 || bdb_home | /var/lib/mysql/ || bdb_log_buffer_size | 32768 || bdb_logdir | || bdb_max_lock | 10000 || bdb_shared_data | OFF || bdb_tmpdir | /tmp/ || binlog_cache_size | 32768 || bulk_insert_buffer_size | 8388608 || character_set_client | latin1 || character_set_connection | latin1 || character_set_database | latin1 || character_set_filesystem | binary || character_set_results | latin1 || character_set_server | latin1 || character_set_system | utf8 || character_sets_dir | /usr/share/mysql/charsets/ || collation_connection | latin1_swedish_ci || collation_database | latin1_swedish_ci || collation_server | latin1_swedish_ci || completion_type | 0 || concurrent_insert | 1 || connect_timeout | 5 || datadir | /var/lib/mysql/ || date_format | %Y-%m-%d || datetime_format | %Y-%m-%d %H:%i:%s || default_week_format | 0 || delay_key_write | ON || delayed_insert_limit | 100 || delayed_insert_timeout | 300 || delayed_queue_size | 1000 || div_precision_increment | 4 || keep_files_on_create | OFF || engine_condition_pushdown | OFF || expire_logs_days | 0 || flush | OFF || flush_time | 0 || ft_boolean_syntax | + -><()~*:""&| || ft_max_word_len | 84 || ft_min_word_len | 4 || ft_query_expansion_limit | 20 || ft_stopword_file | (built-in) || group_concat_max_len | 1024 || have_archive | NO || have_bdb | YES || have_blackhole_engine | NO || have_compress | YES || have_crypt | YES || have_csv | NO || have_dynamic_loading | YES || have_example_engine | NO || have_federated_engine | NO || have_geometry | YES || have_innodb | YES || have_isam | NO || have_merge_engine | YES || have_ndbcluster | NO || have_openssl | DISABLED || have_ssl | DISABLED || have_query_cache | YES || have_raid | NO || have_rtree_keys | YES || have_symlink | YES || hostname | localhost || init_connect | || init_file | || init_slave | || innodb_additional_mem_pool_size | 1048576 || innodb_autoextend_increment | 8 || innodb_buffer_pool_awe_mem_mb | 0 || innodb_buffer_pool_size | 8388608 || innodb_checksums | ON || innodb_commit_concurrency | 0 || innodb_concurrency_tickets | 500 || innodb_data_file_path | ibdata1:10M:autoextend || innodb_data_home_dir | || innodb_doublewrite | ON || innodb_fast_shutdown | 1 || innodb_file_io_threads | 4 || innodb_file_per_table | OFF || innodb_flush_log_at_trx_commit | 1 || innodb_flush_method | || innodb_force_recovery | 0 || innodb_lock_wait_timeout | 50 || innodb_locks_unsafe_for_binlog | OFF || innodb_log_arch_dir | || innodb_log_archive | OFF || innodb_log_buffer_size | 1048576 || innodb_log_file_size | 5242880 || innodb_log_files_in_group | 2 || innodb_log_group_home_dir | ./ || innodb_max_dirty_pages_pct | 90 || innodb_max_purge_lag | 0 || innodb_mirrored_log_groups | 1 || innodb_open_files | 300 || innodb_rollback_on_timeout | OFF || innodb_support_xa | ON || innodb_sync_spin_loops | 20 || innodb_table_locks | ON || innodb_thread_concurrency | 8 || innodb_thread_sleep_delay | 10000 || interactive_timeout | 28800 || join_buffer_size | 131072 || key_buffer_size | 8388600 || key_cache_age_threshold | 300 || key_cache_block_size | 1024 || key_cache_division_limit | 100 || language | /usr/share/mysql/english/ || large_files_support | ON || large_page_size | 0 || large_pages | OFF || lc_time_names | en_US || license | GPL || local_infile | ON || locked_in_memory | OFF || log | OFF || log_bin | OFF || log_bin_trust_function_creators | OFF || log_error | || log_queries_not_using_indexes | OFF || log_slave_updates | OFF || log_slow_queries | OFF || log_warnings | 1 || long_query_time | 10 || low_priority_updates | OFF || lower_case_file_system | OFF || lower_case_table_names | 0 || max_allowed_packet | 1048576 || max_binlog_cache_size | 4294967295 || max_binlog_size | 1073741824 || max_connect_errors | 10 || max_connections | 100 || max_delayed_threads | 20 || max_error_count | 64 || max_heap_table_size | 16777216 || max_insert_delayed_threads | 20 || max_join_size | 4294967295 || max_length_for_sort_data | 1024 || max_prepared_stmt_count | 16382 || max_relay_log_size | 0 || max_seeks_for_key | 4294967295 || max_sort_length | 1024 || max_sp_recursion_depth | 0 || max_tmp_tables | 32 || max_user_connections | 0 || max_write_lock_count | 4294967295 || multi_range_count | 256 || myisam_data_pointer_size | 6 || myisam_max_sort_file_size | 2147483647 || myisam_recover_options | OFF || myisam_repair_threads | 1 || myisam_sort_buffer_size | 8388608 || myisam_stats_method | nulls_unequal || net_buffer_length | 16384 || net_read_timeout | 30 || net_retry_count | 10 || net_write_timeout | 60 || new | OFF || old_passwords | ON || open_files_limit | 1024 || optimizer_prune_level | 1 || optimizer_search_depth | 62 || pid_file | /var/run/mysqld/mysqld.pid || port | 3306 || preload_buffer_size | 32768 || profiling | OFF || profiling_history_size | 15 || protocol_version | 10 || query_alloc_block_size | 8192 || query_cache_limit | 1048576 || query_cache_min_res_unit | 4096 || query_cache_size | 0 || query_cache_type | ON || query_cache_wlock_invalidate | OFF || query_prealloc_size | 8192 || range_alloc_block_size | 2048 || read_buffer_size | 131072 || read_only | OFF || read_rnd_buffer_size | 262144 || relay_log_purge | ON || relay_log_space_limit | 0 || rpl_recovery_rank | 0 || secure_auth | OFF || secure_file_priv | || server_id | 0 || skip_external_locking | ON || skip_networking | OFF || skip_show_database | OFF || slave_compressed_protocol | OFF || slave_load_tmpdir | /tmp/ || slave_net_timeout | 3600 || slave_skip_errors | OFF || slave_transaction_retries | 10 || slow_launch_time | 2 || socket | /var/lib/mysql/mysql.sock || sort_buffer_size | 2097144 || sql_big_selects | ON || sql_mode | || sql_notes | ON || sql_warnings | OFF || ssl_ca | || ssl_capath | || ssl_cert | || ssl_cipher | || ssl_key | || storage_engine | MyISAM || sync_binlog | 0 || sync_frm | ON || system_time_zone | CST || table_cache | 64 || table_lock_wait_timeout | 50 || table_type | MyISAM || thread_cache_size | 0 || thread_stack | 196608 || time_format | %H:%i:%s || time_zone | SYSTEM || timed_mutexes | OFF || tmp_table_size | 33554432 || tmpdir | /tmp/ || transaction_alloc_block_size | 8192 || transaction_prealloc_size | 4096 || tx_isolation | REPEATABLE-READ || updatable_views_with_limit | YES || version | 5.0.51a || version_bdb | Sleepycat Software: Berkeley DB 4.1.24: (January 11, 2008) || version_comment | Source distribution || version_compile_machine | i386 || version_compile_os | redhat-linux-gnu || wait_timeout | 28800 |+---------------------------------+------------------------------------------------------------+234 rows in set (0.00 sec)