Updating more than one column
The traditional advice for improving performance for multiple UPDATE statements is to “prepare” the required query once, and then “execute” the prepared query once for each row requiring an update.
But in many cases this only provides a modest improvement as each UPDATE operation still requires a round-trip communication with the database server.
For small numbers of rows requiring updates, it can be adequate to use an UPDATE statement for each row that requires an update.
But if there are a large number of rows that require an update, then the overhead of issuing large numbers of UPDATE statements can result in the operation as a whole taking a long time to complete.
If you access a column from the table to be updated in an expression, assignments are generally evaluated from left to right.
For multiple-table updates, there is no guarantee that assignments are carried out in any particular order.
In some databases, such as Postgre SQL, when a FROM clause is present, what essentially happens is that the target table is joined to the tables mentioned in the fromlist, and each output row of the join represents an update operation for the target table.
A requirement arises in many systems to update multiple SQL database rows.The implicit default value is tables for which there are foreign key constraints, the My SQL optimizer might process tables in an order that differs from that of their parent/child relationship. The modification of data that is already in the database is referred to as updating.We are going to update column DESCRIPTION and PICTURE in table Categories_Test by using data in table Categories, based on data in the common column CATEGORY_ID. Update data in a column in table A based on a common column in table B. Update data in two columns in table A based on a common column in table B. Conditionally update data in table A based on a common column in table B. No portion may be reproduced without my written permission.
The update query below shows that the PICTURE column is updated by looking up the same ID value in CATEGORY_ID column in table Categories_Test and Categories. If you need to update multiple columns simultaneously, use comma to separate each column after the SET keyword. Here we only want to update PICTURE column in Categories_Test table where the data in Category_Name column is Seafood in table Categories. Software and hardware names mentioned on this site are registered trademarks of their respective companies.Therefore it is not always possible to directly specify which row to update.