使用sql语句查询相应的表结构信息及索引字段等信息
常用的命令有如下：

desc tableName; desc employees.employees;
show columns from tableName; show COLUMNS from employees.employees;
describe tableName; DESCRIBE employees.employees;
这三个显示的结果都是一样的，显示表中filed，type，null，key，default及extra。

show create table tableName; show CREATE TABLE employees.employees;
这个语句会显示这个表的建表语句。

select * from columns where table_name='表名';
select * from information_schema.COLUMNS where TABLE_SCHEMA='employees' and TABLE_NAME='employees';
这个显示的结果就比较全了。

接下来，来点更全的sql，这个是用来同步mysql和orac数据字典的所有sql。

mysql部分：

## 查看所有的库SELECT	lower(schema_name) schema_nameFROM	information_schema.schemataWHERE	schema_name NOT IN (		'mysql',		'information_schema',		'test',		'search',		'tbsearch',		'sbtest',		'dev_ddl'	)## 产看某一个库中的所有表SELECT	table_name,	create_time updated_at,	table_type,	ENGINE,	table_rows num_rows,	table_comment,	ceil(data_length / 1024 / 1024) store_capacityFROM	information_schema.TABLESWHERE	table_schema = 'employees'AND table_name NOT LIKE 'tmp#_%' ESCAPE '#'##查看某一个库下某一个表的所有字段SELECT	lower(column_name) column_name,	ordinal_position position,	column_default dafault_value,	substring(is_nullable, 1, 1) nullable,	column_type data_type,	column_comment,	character_maximum_length data_length,	numeric_precision data_precision,	numeric_scale data_scaleFROM	information_schema.COLUMNSWHERE	table_schema = 'employees'AND table_name = 'employees';## 查看某一个库下某一张表的索引SELECT DISTINCT	lower(index_name) index_name,	lower(index_type) typeFROM	information_schema.statisticsWHERE	table_schema = 'employees'AND table_name = 'employees';## 查看某一个库下某一张表的某一个索引SELECT	lower(column_name) column_name,	seq_in_index column_positionFROM	information_schema.statisticsWHERE	table_schema = 'employees'AND table_name = 'employees'AND index_name = 'primary';## 查看某一个库下某一个表的注释SELECT	table_comment commentsFROM	information_schema.TABLESWHERE	table_schema = 'employees'AND table_name = 'employees';## 查看某一个库下某一个表的列的注释SELECT	lower(column_name) column_name,	column_comment commentsFROM	COLUMNSWHERE	table_schema = 'employees'AND table_name = 'employees';

oracle部分：

#table structure:SELECT	lower(table_name) table_name,	TEMPORARY,	tablespace_name,	num_rows,	duration,	'ORACLE' table_type,	partitioned,	(		SELECT			ceil(sum(bytes) / 1024 / 1024)		FROM			dba_segments b		WHERE			a. OWNER = b. OWNER		AND a.table_name = b.segment_name	) AS store_capacityFROM	dba_tables aWHERE	OWNER = ?AND table_name NOT LIKE 'TMP%';SELECT	lower(column_name) column_name,	column_id position,	data_type,	data_length,	data_precision,	data_scale,	nullable,	data_default default_value,	default_lengthFROM	dba_tab_columnsWHERE	OWNER = ?AND table_name = ?;# indexSELECT	lower(index_name) index_name,	index_type typeFROM	dba_indexesWHERE	OWNER = ?AND table_name = ?AND index_name NOT LIKE 'SYS_IL%';SELECT	lower(column_name) column_name,	column_position,	descendFROM	dba_ind_columnsWHERE	table_owner = ?AND table_name = ?AND index_name = ?;#collect descriptionSELECT	commentsFROM	dba_tab_commentsWHERE	OWNER = ?AND table_name = ?;SELECT	lower(column_name) column_name,	commentsFROM	dba_col_commentsWHERE	OWNER = ?AND table_name = ?;#databaseSELECT	lower(username) usernameFROM	dba_usersWHERE	username NOT IN (		'STDBYPERF',		'READONLY',		'APPQOSSYS',		'ANYSQL',		'DBFLASH',		'SYS',		'SYSTEM',		'MONITOR',		'TBSEARCH',		'MANAGER',		'SYSMAN',		'EXFSYS',		'WMSYS',		'DIP',		'TSMSYS',		'ORACLE_OCM',		'OUTLN',		'DBSNMP',		'PERFSTAT',		'SEARCH',		'TOOLS',		'TBDUMP',		'DMSYS',		'XDB',		'ANONYMOUS',		'DEV_DDL'	);#segsizeSELECT	round(sum(bytes) / 1024 / 1024, 0) mbytesFROM	dba_segmentsWHERE	OWNER = ?AND segment_name = ?;

关于oralce中的segements，可以参考一下这个系列文章。
http://book.51cto.com/art/201108/288137.htm

总结一下，mysql中查看库表字段信息都在information_schemal中，这些是获取数据字典的必备sql。本文中mysql的语句都在本地测试过。另外oracle的结构也要熟悉。