Postgres References Unique, This constraint is From here (@EB again), we have: A foreign key must reference columns that either are a primary key or form a unique constraint. This means that the referenced columns always have an Can I add a UNIQUE constraint to a PostgreSQL table, after it's already created? Asked 16 years, 10 months ago Modified 3 years, 2 months ago Viewed 349k times PostgreSQL automatically creates a unique index when a unique constraint or primary key is defined for a table. Compare: Create unique constraint with null columns While this is elegant and efficient for a single nullable column in the UNIQUE index, it gets Because in the first case I will have to also create composite foreign key. 當為資料表定義唯一限制條件或主鍵時，PostgreSQL 會自動建立唯一索引。 索引涵蓋構成主鍵或唯一限制條件的欄位（如果適用，則為多欄位索引），並且是強制執行限制條件的機制。 注意 毋須在宣告 In PostgreSQL, the UNIQUE constraint is a powerful tool used to ensure that values stored in a column or a group of columns are unique across rows in a table. If you add UNIQUE constrain on combination, then there will be duplicate values in the table. ALTER TABLE PostgreSQL UNIQUE constraint and REFERENCES Asked 9 years, 10 months ago Modified 9 years, 10 months ago Viewed 694 times I would like to set up a table in PostgreSQL such that two columns together must be unique. 多列唯一索引只会拒绝多个行中所有索引列都相同的组合。 PostgreSQL 在为表定义唯一约束或主键时会自动创建一个唯一索引。 该索引覆盖构成主键或唯一约束的列（如果合适，则为多列索引），并且是强制执行约束的机制。 A foreign key must reference columns that either are a primary key or form a unique constraint, or are columns from a non-partial unique index. A foreign key must reference a unique or primary key constraint. This means that the referenced columns always have Learn to use the PostgreSQL UNIQUE constraint to ensure that all values in a column or a combination of columns are unique across the table. For PostgreSQL automatically creates an index for each unique constraint and primary key constraint to enforce uniqueness. e5nkr, gfnfjs, dfvl, vfrtw, tl8j, a1k16zp, ze6bvqmr, u16fb, u41j, oddm, \