My sql update command

The second assignment in the following statement sets col2 to the current updated col1 value, not the original col1 value. The result is that col1 and col2 have the same value. This behavior differs from standard SQL.

For multiple-table updates, there is no guarantee that assignments are carried out in any particular order. If you set a column to the value it currently has, MySQL notices this and does not update it. For information about generated columns, see Section This can be useful in certain situations that might otherwise result in an error.

Suppose that a table t contains a column id that has a unique index. The following statement could fail with a duplicate-key error, depending on the order in which rows are updated:. For example, if the table contains 1 and 2 in the id column and 1 is updated to 2 before 2 is updated to 3, an error occurs.

Its syntax is described in Section Here is an example:. In this case, the statement fails and rolls back. You cannot update a table and select directly from the same table in a subquery. You can work around this by using a multi-table update in which one of the tables is derived from the table that you actually wish to update, and referring to the derived table using an alias. Suppose you wish to update a table named items which is defined using the statement shown here:.

As shown here, this statement does not work:. Instead, you can employ a multi-table update in which the subquery is moved into the list of tables to be updated, using an alias to reference it in the outermost WHERE clause, like this:. Because the optimizer tries by default to merge the derived table discounted into the outermost query block, this works only if you force materialization of the derived table.

In this case, the subquery is materialized by default rather than merged, so it is not necessary to disable merging of the derived table. Data Definition Statements. Atomic Data Definition Statement Support. Now we will execute the following query and then will analyze it. After the execution of the update from a select statement the output of the table will be as below;.

As we can see, the PersonCityName column data of the Persons table have been updated with the City column data of the AddressList table for the matched records for the PersonId column.

Regarding this method, we should underline the following significant points. If the subquery finds more than one matched row, the update query will return an error, as shown below:. In this article, we learned to update the data in a table with the data where they are contained in other tables.

PersonId INT. PersonName , PersonLastName. N 'Salvador' , N 'Williams' ,. N 'Lawrence' , N 'Brown' ,. N 'Gilbert' , N 'Jones' ,. N 'Ernest' , N 'Smith' ,. N 'Jorge' , N 'Johnson'. PersonId , PostCode , City. City ,. AddressList Addr. ON Per. ON Addr. PostCode ,. City ;. PostCode ;. SET Persons. Author Recent Posts.

For example, the following statement updates both last name and email columns of employee number The following example updates the domain parts of emails of all Sales Reps with office code 6 :. For example, in the customers table, some customers do not have any sale representative. To do this, we can select a random employee whose job title is Sales Rep from the employees table and update it for the employees table. This query selects a random employee from the table employees whose job title is the Sales Rep.