UPDATE is a DML statement that modifies rows in a table.

Syntax

UPDATE table_references
    SET column_reference=expr [ , column_reference=expr , ... ]
    [WHERE where_condition]
    [LIMIT row_count];

Arguments

table_references

One or more tables to reference during the update operation. Refer to the SELECT statement documentation for full definition of table_references. The table_references clause may be a comma-separated list of tables, or a join expression. A single table must be the target of the update operation. The target fields must either be identified by two part names (table.field), on the left of assignments in the SET clause, or single identifiers. If single identifiers are used, the target table is assumed to be the leftmost table in the table_references clause.

column_reference

A column in the specified target table to update, designated by either a single identifier or table.field.

expr

An expression that evaluates to a valid column-type value for the specified column.

where_condition

One or more expressions that evaluate to true for each row to update.

row_count

The maximum number of rows to be updated.

Remarks

The UPDATE statement modifies each specified field in a row with new values. Its SET clause indicates which columns to modify and the values they should be given. Each value can be specified as an expression. If specified, the WHERE clause provides any conditions that identify which rows to update. If a WHERE clause is not specified, all rows are updated. Finally, the LIMIT clause places a limit on the number of rows that can be updated.

Although UPDATE supports referencing multiple tables using either join or subquery, MemSQL only supports changing one table in UPDATE statement. In the SET clause, all columns must come from the target table only.

The first field in the target table of type TIMESTAMP or TIMESTAMP(6) has special behavior for update operations, where it is updated to the current timestamp value when not explicitly updated with a different value. Refer to the discussion of these types in the Data Types topic for more details.

In addition, a field f declared with the ON UPDATE <value> modifier will be updated to <value> if any other field is updated, but f itself is not updated.

UPDATE queries will fail if the maximum_table_memory limit has been reached.

This command must be run on the master aggregator or a child aggregator node. For more information, see Node Requirements for MemSQL Commands. Note that when running this command on reference tables you must connect to the master aggregator.

Updating columns which are part of the SHARD key is unsupported.

Writing to multiple databases in a transaction is not supported.

Memsql does not currently support updating a reference table based on a join with a sharded table and returns the following error: ERROR 1706 (HY000): Feature 'Multi-table UPDATE/DELETE with a reference table as target table' is not supported by MemSQL. This is because reference tables can only be updated on the master aggregator while joins against sharded tables must run on the leaves. For example, with t1 as a reference table, the following update will return an error:

UPDATE t1 JOIN t2 ON t1.b = t2.b SET t1.b = t1.b+1;

To avoid getting an error:

• Use the same type of table for update. For example, use both the target table and joined table either as a sharded table or as a reference table.

• Use UPSERT with a reference table that has a primary key. For example, if t1 is a reference table with a primary key, then the following query runs successfully:

  INSERT INTO t1 SELECT t1.* FROM t1 JOIN t2 ON t1.b = t2.b ON DUPLICATE KEY UPDATE t1.b.
  

Examples

UPDATE a SET c1 = 0;

UPDATE a SET c1 = 0 WHERE c2 = 100;

UPDATE a, b SET a.v = b.v WHERE a.name = b.name;

UPDATE a LEFT JOIN b ON a.name = b.name SET a.v = b.v;

UPDATE looooooooong as a, b SET a.v = b.v WHERE a.name = b.name;

UPDATE a, b, c SET a.v = 0 WHERE a.x = b.x and b.y = c.y;

UPDATE a, b, c SET a.v = c.v WHERE a.x = b.x and b.y = c.y;

UPDATE b, a SET a.v = b.v WHERE a.name = b.name;

UPDATE dataset SET valid = false WHERE v = (SELECT MAX(v) FROM dataset);

UPDATE dataset SET valid = false WHERE name IN (SELECT * FROM invalid_names);

UPDATE dataset SET v = v - (SELECT MIN(v) FROM dataset);

UPDATE records a JOIN
  (SELECT name, COUNT(*) as count FROM samples GROUP BY name) b
  SET a.count = a.count + b.count WHERE a.name = b.name;

Updates where more than one value maps to a target row

Depending on the data, some update statements may try to assign more than one value to a single target row. These updates are sometimes called non-deterministic updates or non-functional updates. Such updates are allowed, but the system only guarantees that one of the source values that maps to the target row will be chosen. Which value is chosen is not specified.

Example non-deterministic update The following example shows the behavior of a non-deterministic update.

CREATE TABLE t1(a int, b int);
CREATE TABLE t2(b int, c int);

INSERT t1 VALUES (1,2);
INSERT t2 VALUES (2,3), (2,4);

UPDATE t1, t2 SET t1.b = t2.c WHERE t1.b = t2.b;
Rows matched: 1  Changed: 1  Warnings: 0

SELECT * FROM t1;
+------+------+
| a    | b    |
+------+------+
|    1 |    3 |
+------+------+

The target row’s b field is set to 3, but it could have been set to 4 as well. Which value will be chosen is implementation-defined and unspecified. In general, even different executions of the same command with the same data may produce different results when a non-deterministic update is specified.

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