数据字典实例详解（做一个简单的网页查询数据库）

数据字典可以帮助开发人员理解各个数据项目的类型、数值和它们与现实世界中的对象的关系。做数据库设计时数据字典是不可或缺的一部分，本文列出了几种常用数据的相关已有表获取数据字典的一些脚本，以下脚本仅仅测试了部分数据库版本，未必全部适配。

SqlServer2000：

        select 
          d.name as tname,
	        字段名 = a.name,
	        类型 = b.name,
	        长度 = columnproperty(a.id, a.name, 'PRECISION'),
	        小数位数 = isnull(columnproperty(a.id, a.name, 'Scale'),0),
	        允许空 = case when a.isnullable = 1 then '√' else '' end,
	        默认值 = isnull(e.text, ''),
	        字段说明 = isnull(g.[value], '')
        from syscolumns a 
        left join systypes b on a.xtype = b.xusertype 
        inner join sysobjects d on a.id = d.id and d.xtype = 'U' and d.name <> 'dtproperties'
        left join syscomments e on a.cdefault = e.id 
        left join sysproperties g on a.id = g.id and a.colid = g.smallid       
        order by a.id, a.colorder

SqlServer2005以上：

        select 
          d.name as tname,
	        字段名 = a.name,
	        类型 = b.name,
	        长度 = columnproperty(a.id, a.name, 'PRECISION'),
	        小数位数 = isnull(columnproperty(a.id, a.name, 'Scale'),0),
	        允许空 = case when a.isnullable = 1 then '√' else '' end,
	        默认值 = isnull(e.text, ''),
	        字段说明 = isnull(g.[value], '')
        from syscolumns a 
        left join systypes b on a.xtype = b.xusertype 
        inner join sysobjects d on a.id = d.id and d.xtype = 'U' and d.name <> 'dtproperties'
        left join syscomments e on a.cdefault = e.id 
        left join sys.extended_properties g on a.id = g.major_id and a.colid = g.minor_id 
        order by a.id, a.colorder  

Oracle：

        select 
          a.table_name as tname,
	        a.column_name as 字段名, 
	        a.data_type as 类型,
	        a.data_length as 长度,
	        case when a.data_scale = null then 0 else a.data_scale end as 小数位数,
	        case when a.nullable = 'Y' then '√' else '' end as 允许空,
	        a.data_default as 默认值,
	        case when b.comments = null then '' else b.comments end as 字段说明
        from user_tab_columns A 
        left join user_col_comments B on A.table_name = B.table_name 
        and A.column_name = B.column_name 
        order by column_id   

MySql：

        select 
          a.table_name as tname,
          a.column_name as 字段名, 
          a.data_type as 类型,
          a.character_maximum_length as 长度,
          a.numeric_scale as 小数位数,
          case when a.is_nullable = 'YES' then '√' else '' end as 允许空,
          a.column_default as 默认值,
          column_comment as 字段说明
        from information_schema.columns A 
        order by ordinal_position 

达梦6：

        select 
          d.name as tname,
	        a.name as 字段名,
	        a.TYPE as 类型,
	        a.LENGTH as 长度,
	        a.SCALE as 小数位数,
	        case when a.NULLABLE = 'Y' then '√' else '' end as 允许空,
	        isnull(a.DEFVAL, '') as 默认值,
	        isnull(a.resvd5, '') as 字段说明
        from SYSDBA.SYSCOLUMNS a 
        inner join SYSDBA.SYSTABLES d on a.id = d.id and d.type = 'U'
        order by a.id, a.colid   

达梦7：

        select 
          a.table_name as tname,
	        a.column_name as 字段名, 
	        a.data_type as 类型,
	        a.data_length as 长度,
	        case when a.data_scale = null then 0 else a.data_scale end as 小数位数,
	        case when a.nullable = 'Y' then '√' else '' end as 允许空,
	        a.data_default as 默认值,
	        case when b.comments = null then '' else b.comments end as 字段说明
        from user_tab_columns A 
        left join user_col_comments B on A.table_name = B.table_name 
        and A.column_name = B.column_name 
        order by column_id      

人大金仓：

        select 
          a.table_name as tname,
	        a.column_name as 字段名, 
	        a.data_type as 类型,
	        a.data_length as 长度,
	        case when a.data_scale = null then 0 else a.data_scale end as 小数位数,
	        case when a.nullable = 'Y' then '√' else '' end as 允许空,
	        a.data_default as 默认值,
	        '' as 字段说明 --相关的字段说明没有找到怎么获取
        from user_tab_columns A 
        order by column_id   

当前的数据库设计更倾向于直接在数据库里建数据字典表，就不存在数据库兼容适配的问题了。