24.35 SHOW 语句的扩展
SHOW语句的某些扩展伴随INFORMATION_SCHEMA的实现：
如果一个视图依赖一个或多个其他视图，并且这些基础视图之一已更新，则IS_UPDATABLE标志可能不可靠。无论IS_UPDATABLE值如何，服务器都会跟踪视图的可更新性，并正确拒绝对不可更新的视图的数据更改操作。如果由于基础视图的更改而导致视图的IS_UPDATABLE值变得不准确，则可以通过删除和重新创建视图来更新该值。
INFORMATION_SCHEMA是一个信息数据库，因此其名称包含在SHOW DATABASES的输出中。同样，SHOW TABLES可以与INFORMATION_SCHEMA一起使用以获取其 table 的列 table：
mysql> SHOW TABLES FROM INFORMATION_SCHEMA;
+---------------------------------------+
| Tables_in_INFORMATION_SCHEMA |
+---------------------------------------+
| CHARACTER_SETS |
| COLLATIONS |
| COLLATION_CHARACTER_SET_APPLICABILITY |
| COLUMNS |
| COLUMN_PRIVILEGES |
| ENGINES |
| EVENTS |
| FILES |
| GLOBAL_STATUS |
| GLOBAL_VARIABLES |
| KEY_COLUMN_USAGE |
| PARTITIONS |
| PLUGINS |
| PROCESSLIST |
| REFERENTIAL_CONSTRAINTS |
| ROUTINES |
| SCHEMATA |
| SCHEMA_PRIVILEGES |
| SESSION_STATUS |
| SESSION_VARIABLES |
| STATISTICS |
| TABLES |
| TABLE_CONSTRAINTS |
| TABLE_PRIVILEGES |
| TRIGGERS |
| USER_PRIVILEGES |
| VIEWS |
+---------------------------------------+
SHOW COLUMNS和DESCRIBE可以显示有关单个INFORMATION_SCHEMAtable 中的列的信息。
接受LIKE子句以限制显示的行的SHOW语句还允许WHERE子句，该子句指定所选行必须满足的更一般的条件：
SHOW CHARACTER SET
SHOW COLLATION
SHOW COLUMNS
SHOW DATABASES
SHOW FUNCTION STATUS
SHOW INDEX
SHOW OPEN TABLES
SHOW PROCEDURE STATUS
SHOW STATUS
SHOW TABLE STATUS
SHOW TABLES
SHOW TRIGGERS
SHOW VARIABLES
WHERE子句(如果存在)将根据SHOW语句显示的列名进行评估。例如，显示字符集语句产生以下输出列：
mysql> SHOW CHARACTER SET;
+----------+-----------------------------+---------------------+--------+
| Charset | Description | Default collation | Maxlen |
+----------+-----------------------------+---------------------+--------+
| big5 | Big5 Traditional Chinese | big5_chinese_ci | 2 |
| dec8 | DEC West European | dec8_swedish_ci | 1 |
| cp850 | DOS West European | cp850_general_ci | 1 |
| hp8 | HP West European | hp8_english_ci | 1 |
| koi8r | KOI8-R Relcom Russian | koi8r_general_ci | 1 |
| latin1 | cp1252 West European | latin1_swedish_ci | 1 |
| latin2 | ISO 8859-2 Central European | latin2_general_ci | 1 |
...
要将WHERE子句与显示字符集一起使用，您将引用这些列名称。例如，以下语句显示有关字符集的信息，其默认排序规则包含字符串'japanese'：
mysql> SHOW CHARACTER SET WHERE `Default collation` LIKE '%japanese%';
+---------+---------------------------+---------------------+--------+
| Charset | Description | Default collation | Maxlen |
+---------+---------------------------+---------------------+--------+
| ujis | EUC-JP Japanese | ujis_japanese_ci | 3 |
| sjis | Shift-JIS Japanese | sjis_japanese_ci | 2 |
| cp932 | SJIS for Windows Japanese | cp932_japanese_ci | 2 |
| eucjpms | UJIS for Windows Japanese | eucjpms_japanese_ci | 3 |
+---------+---------------------------+---------------------+--------+
此语句显示多字节字符集：
mysql> SHOW CHARACTER SET WHERE Maxlen > 1;
+---------+---------------------------+---------------------+--------+
| Charset | Description | Default collation | Maxlen |
+---------+---------------------------+---------------------+--------+
| big5 | Big5 Traditional Chinese | big5_chinese_ci | 2 |
| ujis | EUC-JP Japanese | ujis_japanese_ci | 3 |
| sjis | Shift-JIS Japanese | sjis_japanese_ci | 2 |
| euckr | EUC-KR Korean | euckr_korean_ci | 2 |
| gb2312 | GB2312 Simplified Chinese | gb2312_chinese_ci | 2 |
| gbk | GBK Simplified Chinese | gbk_chinese_ci | 2 |
| utf8 | UTF-8 Unicode | utf8_general_ci | 3 |
| ucs2 | UCS-2 Unicode | ucs2_general_ci | 2 |
| cp932 | SJIS for Windows Japanese | cp932_japanese_ci | 2 |
| eucjpms | UJIS for Windows Japanese | eucjpms_japanese_ci | 3 |
+---------+---------------------------+---------------------+--------+
