如果你忘记一个数据库或表的名字，或一个给定的表的结构是什么(例如，它的列叫什么)，怎么办？ MySQL通过提供数据库及其支持的表的信息的几个语句解决这个问题。
你已经见到了SHOW DATABASES，它列出由服务器管理的数据库。为了找出当前选择了哪个数据库，使用DATABASE()函数：
mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| menagerie |
+------------+
如果你还没选择任何数据库，结果是空的。
为了找出当前的数据库包含什么表(例如，当你不能确定一个表的名字)，使用这个命令：
mysql> SHOW TABLES;
+---------------------+
| Tables in menagerie |
+---------------------+
| event |
| pet |
+---------------------+
如果你想要知道一个表的结构，DESCRIBE命令是有很用的；它显示有关一个表的每个列的信息：
mysql> DESCRIBE pet;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name | varchar(20) | YES | | NULL | |
| owner | varchar(20) | YES | | NULL | |
| species | varchar(20) | YES | | NULL | |
| sex | char(1) | YES | | NULL | |
| birth | date | YES | | NULL | |
| death | date | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
Field显示列名字，Type是为列的数据类型，Null表示列是否能包含NULL值，Key显示列是否被索引而Default指定列的缺省值。
如果你在一个表上有索引，SHOW INDEX FROM tbl_name生成有关它们的信息--表说明
SELECT dbo.sysobjects.name AS TableName,
dbo.sysproperties.[value] AS TableDesc
FROM dbo.sysproperties INNER JOIN
dbo.sysobjects ON dbo.sysproperties.id = dbo.sysobjects.id
WHERE (dbo.sysproperties.smallid = 0)
ORDER BY dbo.sysobjects.name
--字段说明
SELECT dbo.sysobjects.name AS TableName, dbo.syscolumns.colid,
dbo.syscolumns.name AS ColName, dbo.sysproperties.[value] AS ColDesc FROM dbo.sysproperties INNER JOIN
dbo.sysobjects ON dbo.sysproperties.id = dbo.sysobjects.id INNER JOIN
dbo.syscolumns ON dbo.sysobjects.id = dbo.syscolumns.id AND
dbo.sysproperties.smallid = dbo.syscolumns.colid
ORDER BY dbo.sysobjects.name, dbo.syscolumns.colid
--主键、外键信息(简化)
select
c_obj.name as CONSTRAINT_NAME
,t_obj.name as TABLE_NAME
,col.name as COLUMN_NAME
,case col.colid
when ref.fkey1 then 1
when ref.fkey2 then 2
when ref.fkey3 then 3
when ref.fkey4 then 4
when ref.fkey5 then 5
when ref.fkey6 then 6
when ref.fkey7 then 7
when ref.fkey8 then 8
when ref.fkey9 then 9
when ref.fkey10 then 10
when ref.fkey11 then 11
when ref.fkey12 then 12
when ref.fkey13 then 13
when ref.fkey14 then 14
when ref.fkey15 then 15
when ref.fkey16 then 16
end as ORDINAL_POSITION
from
sysobjects c_obj
,sysobjects t_obj
,syscolumns col
,sysreferences ref
where
permissions(t_obj.id) != 0
and c_obj.xtype in ('F ')
and t_obj.id = c_obj.parent_obj
and t_obj.id = col.id
and col.colid in
(ref.fkey1,ref.fkey2,ref.fkey3,ref.fkey4,ref.fkey5,ref.fkey6,ref.fkey7,ref.fkey8,ref.fkey9,ref.fkey10,ref.fkey11,ref.fkey12,ref.fkey13,ref.fkey14,ref.fkey15,ref.fkey16)
and c_obj.id = ref.constid
union
select
i.name as CONSTRAINT_NAME
,t_obj.name as TABLE_NAME
,col.name as COLUMN_NAME
,v.number as ORDINAL_POSITION
from
sysobjects c_obj
,sysobjects t_obj
,syscolumns col
,master.dbo.spt_values v
,sysindexes i
where
permissions(t_obj.id) != 0
and c_obj.xtype in ('UQ' ,'PK')
and t_obj.id = c_obj.parent_obj
and t_obj.xtype = 'U'
and t_obj.id = col.id
and col.name = index_col(t_obj.name,i.indid,v.number)
and t_obj.id = i.id
and c_obj.name = i.name
and v.number > 0
and v.number <= i.keycnt
and v.type = 'P'
order by CONSTRAINT_NAME, ORDINAL_POSITION
--主键、外键对照(简化)
select
fc_obj.name as CONSTRAINT_NAME
,i.name as UNIQUE_CONSTRAINT_NAME
from
sysobjects fc_obj
,sysreferences r
,sysindexes i
,sysobjects pc_obj
where
permissions(fc_obj.parent_obj) != 0
and fc_obj.xtype = 'F'
and r.constid = fc_obj.id
and r.rkeyid = i.id
and r.rkeyindid = i.indid
and r.rkeyid = pc_obj.id
----------------- ORACLE -------------------
--表信息
select * from all_tab_comments t
where owner='DBO'
--列信息
select * from all_col_comments t
where owner='DBO'
--主键、外键对照
select OWNER, CONSTRAINT_NAME, CONSTRAINT_TYPE, TABLE_NAME, R_OWNER, R_CONSTRAINT_NAME
from all_constraints
where owner='DBO' and (Constraint_Type='P' or Constraint_Type='R')
--主键、外键信息
select *
from all_cons_columns
where owner='DBO'
order by Constraint_Name, Position
------------------------- Access ------------------------
//Access中的系统表MSysobjects存储属性的字段是二进制格式，不能直接分析可以采用ADO自带的OpenSchema方法获得相关信息
//use ADOInt.pas
//po: TableName
//DBCon:TADOConnection
/ds:TADODataSet
--表信息
DBCon.OpenSchema(siTables, VarArrayOf([Null, Null, 'Table']), EmptyParam, ds);
--列信息
DBCon.OpenSchema(siColumns, VarArrayOf([Null, Null, 'po']), EmptyParam, ds);
--主键
DBCon.OpenSchema(siPrimaryKeys, EmptyParam, EmptyParam, ds);
--主键、外键对照
DBCon.OpenSchema(siForeignKeys, EmptyParam, EmptyParam, ds);