UPSERT is a clause added to INSERT that causes the INSERT to behave as an UPDATE or a no-op if the INSERT would violate a uniqueness constraint. UPSERT is not standard SQL. UPSERT in SQLite follows the syntax established by PostgreSQL, with generalizations.
An UPSERT is an ordinary INSERT statement that is followed by one or more ON CONFLICT clauses, as shown in the syntax diagram above.
The syntax in between the "ON CONFLICT" and "DO" keywords is called the "conflict target". The conflict target specifies a uniqueness constraint that will trigger the upsert. The conflict target may be omitted on the last ON CONFLICT clause in the INSERT statement, but is required for all other ON CONFLICT clauses.
If the insert operation would cause the conflict target uniqueness constraint to fail, then the insert is omitted and the corresponding DO NOTHING or DO UPDATE operation is performed instead. The ON CONFLICT clauses are checked in the order specified. If the last ON CONFLICT clause omits the conflict target, then it will fire if any uniqueness constraint fails which is not captured by prior ON CONFLICT clauses.
Only a single ON CONFLICT clause, specifically the first ON CONFLICT clause with a matching conflict target, may run for each row of the INSERT. When an ON CONFLICT clause fires, all subsequent ON CONFLICT clauses are bypassed for that one row.
In the case of a multi-row insert, the upsert decision is made separately for each row of the insert.
The UPSERT processing happens only for uniqueness constraints. A "uniqueness constraint" is an explicit UNIQUE or PRIMARY KEY constraint within the CREATE TABLE statement, or a unique index. UPSERT does not intervene for failed NOT NULL, CHECK, or foreign key constraints or for constraints that are implemented using triggers.
Column names in the expressions of a DO UPDATE refer to the original unchanged value of the column, before the attempted INSERT. To use the value that would have been inserted had the constraint not failed, add the special "excluded." table qualifier to the column name.
Some examples will help illustrate how UPSERT works:
CREATE TABLE vocabulary(word TEXT PRIMARY KEY, count INT DEFAULT 1); INSERT INTO vocabulary(word) VALUES('jovial') ON CONFLICT(word) DO UPDATE SET count=count+1;
The upsert above inserts the new vocabulary word "jovial" if that word is not already in the dictionary, or if it is already in the dictionary, it increments the counter. The "count+1" expression could also be written as "vocabulary.count". PostgreSQL requires the second form, but SQLite accepts either.
CREATE TABLE phonebook(name TEXT PRIMARY KEY, phonenumber TEXT); INSERT INTO phonebook(name,phonenumber) VALUES('Alice','704-555-1212') ON CONFLICT(name) DO UPDATE SET phonenumber=excluded.phonenumber;
In the second example, the expression in the DO UPDATE clause is of the form "excluded.phonenumber". The "excluded." prefix causes the "phonenumber" to refer to the value for phonenumber that would have been inserted had there been no conflict. Hence, the effect of the upsert is to insert a phonenumber of Alice if none exists, or to overwrite any prior phonenumber for Alice with the new one.
Note that the DO UPDATE clause acts only on the single row that experienced the constraint error during INSERT. It is not necessary to include a WHERE clause that restricts the action to that one row. The only use for the WHERE clause at the end of the DO UPDATE is to optionally change the DO UPDATE into a no-op depending on the original and/or new values. For example:
CREATE TABLE phonebook2( name TEXT PRIMARY KEY, phonenumber TEXT, validDate DATE ); INSERT INTO phonebook2(name,phonenumber,validDate) VALUES('Alice','704-555-1212','2018-05-08') ON CONFLICT(name) DO UPDATE SET phonenumber=excluded.phonenumber, validDate=excluded.validDate WHERE excluded.validDate>phonebook2.validDate;
In this last example, the phonebook2 entry is only updated if the validDate for the newly inserted value is newer than the entry already in the table. If the table already contains an entry with the same name and a current validDate, then the WHERE clause causes the DO UPDATE to become a no-op.
When the INSERT statement to which the UPSERT is attached takes its values from a SELECT statement, there is a potential parsing ambiguity. The parser might not be able to tell if the "ON" keyword is introducing the UPSERT or if it is the ON clause of a join. To work around this, the SELECT statement should always include a WHERE clause, even if that WHERE clause is just "WHERE true".
Ambiguous use of ON:
INSERT INTO t1 SELECT * FROM t2 ON CONFLICT(x) DO UPDATE SET y=excluded.y;
Ambiguity resolved using a WHERE clause:
INSERT INTO t1 SELECT * FROM t2 WHERE true ON CONFLICT(x) DO UPDATE SET y=excluded.y;
UPSERT does not currently work for virtual tables.
The conflict resolution algorithm for the update operation of the DO UPDATE clause is always ABORT. In other words, the behavior is as if the DO UPDATE clause were actually written as "DO UPDATE OR ABORT". If the DO UPDATE clause encounters any constraint violation, the entire INSERT statement rolls back and halts. This is true even if the DO UPDATE clause is contained within an INSERT statement or a trigger that specifies some other conflict resolution algorithm.
UPSERT syntax was added to SQLite with version 3.24.0 (2018-06-04). The original implementation closely followed the PostgreSQL syntax in that it only permitted a single ON CONFLICT clause and it required a conflict target for on DO UPDATE. The syntax was generalized to permit multiple ON CONFLICT clauses and to allow DO UPDATE resolution without a conflict target in SQLite version 3.35.0 (2021-03-12).
This page last modified on 2022-01-08 05:02:57 UTC
*** DRAFT ***