今天需要一个查询Oracle某张表全部唯一约束字段的功能，上网查了一下，找到一段sql，查询的user_constraints表，试了下查询不到我要的表的唯一约束，接着查发现全都是复制粘贴的一样的，后来找了下JDBC的DatabaseMetaData类，发现了一个getIndexInfo方法，是可以查到唯一约束的，但是会有一个null，而且type是0和1，出于好奇翻了下Oracle的JDBC驱动源码，发现Oracle并不是查询的user_constraints表，下面是Oracle驱动中的sql

select null as table_cat,
       owner as table_schem,
       table_name,
       0 as NON_UNIQUE,
       null as index_qualifier,
       null as index_name, 0 as type,
       0 as ordinal_position, null as column_name,
       null as asc_or_desc,
       num_rows as cardinality,
       blocks as pages,
       null as filter_condition
from all_tables
where table_name = '表名'
  and owner = 'schema名'
union
select null as table_cat,
       i.owner as table_schem,
       i.table_name,
       decode (i.uniqueness, 'UNIQUE', 0, 1),
       null as index_qualifier,
       i.index_name,
       1 as type,
       c.column_position as ordinal_position,
       c.column_name,
       null as asc_or_desc,
       i.distinct_keys as cardinality,
       i.leaf_blocks as pages,
       null as filter_condition
from all_indexes i, all_ind_columns c
where i.table_name = '表名'
  and i.owner = 'schema名'
  and i.uniqueness = 'UNIQUE'
  and i.index_name = c.index_name
  and i.table_owner = c.table_owner
  and i.table_name = c.table_name
  and i.owner = c.index_owner
order by non_unique, type, index_name, ordinal_position

可以看到它把表作为一个index与UNIQUE约束的查询结果union后做了个排序，所以JDBC中永远会查到一个列名为null的结果，由于我个人不需要这个，因此去掉union半部分和order by即可