what is the difference between unique key constraint and primary key constraint?
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A primary key is a unique field on a table but it is special in the sense that the table considers that row as its key. This means that other tables can use this field to create foreign key relationships to themselves. A unique constraint simply means that a particular field must be unique. Primary keys can't be null.
829300
You can only have 1 primary key in a database table. ... You can have more than 1 unique key table column (unlike primary key which means only 1 table column in the table is unique). INDEX also creates uniqueness. MySQL (example) will create a indexing table for the column that is indexed.
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Update : A candidate key is a unique key that can be used as a primary key. Composite key is a key of two or more attributes that uniquely identifies the row. A key is a set of columns that can be used to uniquely identify each row within a table.
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It uniquely identifies a record in the relational database table. It refers to the field in a table which is the primary key of another table. Only one primary key is allowed in a table. Whereas more than one foreign key are allowed in a table.
816442
A primary key is used to ensure data in the specific column is unique. You can only set constraints with primary keys, by setting a foreign key to another column which creates a relationship with the column that has the primary key set. A prime use of a primary key is in the case of a users table.
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In MySQL, a primary key is a single field or combination of fields that uniquely defines a record. None of the fields that are part of the primary key can contain a NULL value. A table can have only one primary key.
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Primary key allows each row in a table to be uniquely identified and ensures that no duplicate rows exist and no null values are entered. Unique key constraint is used to prevent the duplication of key values within the rows of a table and allow null values. (In oracle, one null is not equal to another null).
1269825
No. Primary Key is a unique and non-null key which identify a record uniquely in table. A table can have only one primary key. Candidate key is also a unique key to identify a record uniquely in a table but a table can have multiple candidate keys.
629041
1. The not null constraint is by default added to primary key, it means, primary key attribute cannot accept null values, whereas, the attribute declared as unique can accept null values. It is the major difference between the two.
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Primary Key: that is used for uniquely identified a perticular row of a database table . Primary INDEX: That is used for getting a index from a database table and that indexing is done on unique based.
395942
Primary key is used to identify data uniquely therefore two rows can't have the same primary key. It can't be null. On the other hand, foreign key is used to maintain relationship between two tables. Primary of a table act as forgein key in the other table.
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- PK: primary key (column is part of a pk) - NN: not null (column is nullable) - UQ: unique (column is part of a unique key) - AI: auto increment (the column is auto incremented when rows are inserted) [additional data type flags, depend on used data type]
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Primary key and foreign key are the two most important and common types of keys used in relational databases. A primary key is a special key used to uniquely identify records in a table, whereas a foreign key is used to establish relationship between two tables.
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Unique and Distinct are two SQL constraints. The main difference between Unique and Distinct in SQL is that Unique helps to ensure that all the values in a column are different while Distinct helps to remove all the duplicate records when retrieving the records from a table.
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Relation of Primary Key vs Foreign Key A primary key uniquely identifies a record in the relational database table, whereas a foreign key refers to the field in a table which is the primary key of another table.
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A primary key is a candidate key that has been selected to be the unique identifier of each tuple in the relation. A foreign key is an attribute of a table that is also a primary key in another table. A composite key is a combination of two fields that together can uniquely identify a record.
1475024
A primary key acts as a unique identifier for each record meaning it uniquely identifies each row/record in a table. "A foreign key" is a field in one table that relates the tables together.