用于创建表的 SQL 查询具有唯一约束，但该列的数据不唯一

Question

create table table1 (
   table_identifier integer identity,
   routing_identifier integer not null,
   account_code char(4) not null),
   constraint pk_table1 primary key (table_identifier),
   constraint ak_table1 unique (routing_identifier, account_code)

I inserted data myself by creating random test data, and the data I have inserted in routing_identifier and account_code have duplicates in them. I didn't notice until now that both of those columns have many duplicates in the test data I created. I'm not sure why it didn't error out stating unique key, no duplicates allowed or something. Does anyone know why this is?

Answer 1

unique (routing_identifier, account_code) means that the combination of routing_identifier and account_code should be unique, not that they are both unique on their own right. Eg, consider these three unique pairs: (1, 10), (2, 10) and (1, 20) - each of these pairs is unique, but neither routing_identifier nor account_code are unique on their own right.

If you want each of these columns to be unique, you should define two separate unique constraints:

create table table1 (
   table_identifier integer identity,
   routing_identifier integer not null,
   account_code char(4) not null),
   constraint pk_table1 primary key (table_identifier),
   constraint ak_routing_identifier_table1 unique (routing_identifier), -- Here!
   constraint ak_account_code_table1 unique (account_code) -- And here!
)