For the purposes of UNIQUE constraints, NULL values are considered distinct from all other values, including other NULLs. For each UNIQUE constraint on the table, each row must contain a unique combination of values in the columns identified by the UNIQUE constraint. So as a constraint, SQLite PK is a synonym for UNIQUE instead of UNIQUE NOT NULL.Ī UNIQUE constraint is similar to a PRIMARY KEY constraint, except that a single table may have any number of UNIQUE constraints. This is like when you declare a column set UNIQUE NULL. But it uses the usual SQL interpretation that NULL is not equal to NULL for purposes of UNIQUE. Since NULL in a PK is against SQL, it seems moot what SQLite then chooses to do when constraining and manipulating tables with NULLs in a PK. Hence, it has been decided to merely document the fact that SQLite allowing NULLs in most PRIMARY KEY columns. If you specify an ON DUPLICATE KEY UPDATE clause and a row to be inserted would cause a duplicate value in a UNIQUE index or PRIMARY KEY, an UPDATE of the old row occurs. SQLite could be fixed to conform to the standard, but doing so might break legacy applications. Unless the column is an INTEGER PRIMARY KEY or the table is a WITHOUT ROWID table or the column is declared NOT NULL, SQLite allows NULL values in a PRIMARY KEY column. ![]() Unfortunately, due to a bug in some early versions, this is not the case in SQLite. The common approach: ON DUPLICATE KEY UPDATE The result of that approach, disturbingly: Insert unless the PRIMARY or any UNIQUE key exists, otherwise update What can go horribly wrong with ON DUPLICATE KEY UPDATE You insert a supposedly new record, with a new PRIMARY key value (say a UUID), but you happen to have a duplicate value for its. SQLite INSERT - ON DUPLICATE KEY UPDATE (UPSERT) Ask Question Asked 13 years, 4 months ago Modified 3 years, 7 months ago Viewed 117k times 113 MySQL has something like this: INSERT INTO visits (ip, hits) VALUES ('127.0.0. ![]() If an INSERT or UPDATE statement attempts to modify the table content so that two or more rows have identical primary key values, that is a constraint violation.Īccording to the SQL standard, PRIMARY KEY should always imply NOT NULL. For the purposes of determining the uniqueness of primary key values, NULL values are considered distinct from all other values, including other NULLs. A unique key is a constraint too, so if you declare a column as UNIQUE or PRIMARY KEY or such, you say 'This column can never contain duplicate values', so when you try to insert a row with the exact same value for that column than an existing row, a constraint failure happens, which is called a 'conflict' and which could be handled in several. You should declare PK columns NOT NULL and not put NULL in them.Įach row in a table with a primary key must have a unique combination of values in its primary key columns. You shouldn't expect to be able to have a NULL in a value for a PK, let alone only one. ![]() SQL PK (PRIMARY KEY) means UNIQUE NOT NULL.
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