查询表或字段是否存在

1、MSSQL Server
表：select COUNT(*) from dbo.sysobjects WHERE name= 'table_name'；
字段：select COUNT(*) from syscolumns WHERE id=object_id(‘table_name’) and name= 'column_name'；
2、My SQL
表：select COUNT(*) from information_schema.tables WHERE table_name = 'table_name'；
字段：select COUNT(*) from information_schema.columns WHERE table_name = 'table_name' and column_name = 'column_name'；
3、Oracle
表：SELECT count(*) FROM USER_OBJECTS WHERE OBJECT_NAME = 'table_name'；
字段：SELECT COUNT(*) FROM USER_TAB_COLUMNS WHERE TABLE_NAME = 'table_name' and column_name = 'column_name'；
4、PostgreSql
表：select count(*) from information_schema.tables WHERE table_schema='table_schema' and table_name ='table_name'；
字段：select count(*) from information_schema.columns WHERE table_schema='table_schema'