（一）Oracle:
1、查询某个表中的字段名称、类型、精度、长度、是否为空
select COLUMN_NAME,DATA_TYPE,DATA_PRECISION,DATA_SCALE,NULLABLE
from user_tab_columns
where table_name ='YourTableName'
2、查询某个表中的主键字段名
select col.column_name
from user_constraints con,  user_cons_columns col
where con.constraint_name = col.constraint_name
and con.constraint_type='P'
and col.table_name = 'YourTableName'
3、查询某个表中的外键字段名称、所引用表名、所应用字段名
select distinct(col.column_name),r.table_name,r.column_name
from
user_constraints con,
user_cons_columns col,
(select t2.table_name,t2.column_name,t1.r_constraint_name
 from user_constraints t1,user_cons_columns t2
 where t1.r_constraint_name=t2.constraint_name
 and t1.table_name='YourTableName'
 ) r
where con.constraint_name=col.constraint_name
and con.r_constraint_name=r.r_constraint_name
and con.table_name='YourTableName'

（二）SQLServer中的实现：
1、字段：
SELECT c.name,t.name,c.xprec,c.xscale,c.isnullable
FROM systypes t,syscolumns c
WHERE t.xtype=c.xtype
AND c.id = (SELECT id FROM sysobjects WHERE name='YourTableName')
ORDER BY c.colid

2、主键（参考SqlServer系统存储过程sp_pkeys）：
select COLUMN_NAME = convert(sysname,c.name)              
from                                                      
sysindexes i, syscolumns c, sysobjects o                  
where o.id = object_id('[YourTableName]')                 
and o.id = c.id                                           
and o.id = i.id                                           
and (i.status & 0x800) = 0x800                            
and (c.name = index_col ('[YourTableName]', i.indid,  1) or    
     c.name = index_col ('[YourTableName]', i.indid,  2) or    
     c.name = index_col ('[YourTableName]', i.indid,  3) or    
     c.name = index_col ('[YourTableName]', i.indid,  4) or    
     c.name = index_col ('[YourTableName]', i.indid,  5) or    
     c.name = index_col ('[YourTableName]', i.indid,  6) or    
     c.name = index_col ('[YourTableName]', i.indid,  7) or    
     c.name = index_col ('[YourTableName]', i.indid,  8) or    
     c.name = index_col ('[YourTableName]', i.indid,  9) or    
     c.name = index_col ('[YourTableName]', i.indid, 10) or    
     c.name = index_col ('[YourTableName]', i.indid, 11) or    
     c.name = index_col ('[YourTableName]', i.indid, 12) or    
     c.name = index_col ('[YourTableName]', i.indid, 13) or    
     c.name = index_col ('[YourTableName]', i.indid, 14) or    
     c.name = index_col ('[YourTableName]', i.indid, 15) or    
     c.name = index_col ('[YourTableName]', i.indid, 16)      
     )

3、外键：
select t1.name,t2.rtableName,t2.name
from
(select col.name, f.constid as temp
 from syscolumns col,sysforeignkeys f
 where f.fkeyid=col.id
 and f.fkey=col.colid
 and f.constid in
 ( select distinct(id) 
   from sysobjects
   where OBJECT_NAME(parent_obj)='YourTableName'
   and xtype='F'
  )
 ) as t1 ,
(select OBJECT_NAME(f.rkeyid) as rtableName,col.name, f.constid as temp
 from syscolumns col,sysforeignkeys f
 where f.rkeyid=col.id
 and f.rkey=col.colid
 and f.constid in
 ( select distinct(id)
   from sysobjects
   where OBJECT_NAME(parent_obj)='YourTableName'
   and xtype='F'
 )
) as t2
where t1.temp=t2.temp