MySql中的變量定義
根據mysql手冊，mysql的變量分爲兩種：系統變量和用戶變量。可是在實際使用中，還會遇到諸如局部變量、會話變量等概念。根據我的感受，mysql變量大致能夠分爲四種類型：mysql
1、局部變量。linux
局部變量通常用在sql語句塊中，好比存儲過程的begin/end。其做用域僅限於該語句塊，在該語句塊執行完畢後，局部變量就消失了。sql
局部變量通常用declare來聲明，能夠使用default來講明默認值。服務器
例如在存儲過程當中定義局部變量：session
drop procedure if exists add;socket
create procedure addoop
(ui
in a int,spa
in b int命令行
)
begin
declare c int default 0;
set c = a + b;
select c as c;
end;
在上述存儲過程當中定義的變量c就是局部變量
2、用戶變量。
用戶變量的做用域要比局部變量要廣。用戶變量能夠做用於當前整個鏈接，可是噹噹前鏈接斷開後，其所定義的用戶變量都會消失。
用戶變量使用以下(這裏咱們無須使用declare關鍵字進行定義，能夠直接這樣使用):
select @變量名
對用戶變量賦值有兩種方式，一種是直接用"="號，另外一種是用":="號。其區別在於使用set命令對用戶變量進行賦值時，兩種方式均可以使用；當使用select語句對用戶變量進行賦值時，只能使用":="方式，由於在select語句中，"="號被看做是比較操做符。
示例程序以下：
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爲用戶變量
+-------+
| @var1 |
+-------+
| 1 |
+-------+
1 row in set (0.00 sec)
mysql> select @var2; //var2爲用戶變量
+-------+
| @var2 |
+-------+
| 2 |
+-------+
1 row in set (0.00 sec)
三、會話變量
服務器爲每一個鏈接的客戶端維護一系列會話變量。在客戶端鏈接時，使用相應全局變量的當前值對客戶端的會話變量進行初始化。設置會話變量不須要特殊權限，但客戶端只能更改本身的會話變量，而不能更改其它客戶端的會話變量。會話變量的做用域與用戶變量同樣，僅限於當前鏈接。噹噹前鏈接斷開後，其設置的全部會話變量均失效。
設置會話變量有以下三種方式：
set session var_name = value;
set @@session.var_name = value;
set var_name = value;
查看一個會話變量也有以下三種方式：
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)
四、全局變量
全局變量影響服務器總體操做。當服務器啓動時，它將全部全局變量初始化爲默認值。這些默認值能夠在選項文件中或在命令行中指定的選項進行更改。要想更改全局變量，必須具備SUPER權限。全局變量做用於server的整個生命週期，可是不能跨重啓。即重啓後全部設置的全局變量均失效。要想讓全局變量重啓後繼續生效，須要更改相應的配置文件。
要設置一個全局變量，有以下兩種方式：
set global var_name = value; //注意：此處的global不能省略。根據手冊，set命令設置變量時若不指定GLOBAL、SESSION或者LOCAL，默認使用SESSION
set @@global.var_name = value; //同上
要想查看一個全局變量，有以下兩種方式：
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)
- 1. MySql中的變量定義
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