Updating fields in sql
Updating fields in sql - Chat dirty time
So is structured so that approaches which are generic across different SQL databases are expressed in a base class, and approaches which only work for specific SQL databases are expressed in a subclass.An object of the relevant class is instantiated when the call is made, and control then passed to the implementation relevant to the database in use.
The SET clause then takes the “salary” field from the “updates” table and uses it to update the “salary” field of the “staff” table.
(It will use placeholders and parameter binding if it thinks it’s appropriate.) If given our second example with two distinct values, will spot that there are two distinct values, 12, and will effect this with two UPDATE statements as described above.
Optimising the number of UPDATEs by grouping the distinct SET values can be done in a way which is compatible with most common SQL databases. FROM approach requires knowledge of the specific SQL database being used.
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.
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.
If there is no database-specific subclass for the database in use, then will just use the base class which implements approaches that should work for any SQL database.
At the time of writing, the only database-specific subclass is for Postgre SQL. Let’s expand the original table a bit: “name” has now been split into “first_name” and “last_name”. If you update values in multiple columns, you use a comma (,) to separate each pair of column and value.The columns that are not on the list retain their original values.So we could think in terms of creating a re-usable module which would implement that logic.This is the intention of UPDATE staff SET salary = 1200 WHERE name = ' Bob'; UPDATE staff SET salary = 1200 WHERE name = ' Jane'; UPDATE staff SET salary = 1200 WHERE name = ' Frank'; UPDATE staff SET salary = 1200 WHERE name = ' Susan'; UPDATE staff SET salary = 1200 WHERE name = ' John'; “key_columns” specifies the columns which will be used to identify rows which need to be updated (using WHERE). The first element provides the value of the column (specified by “key_columns”) to identify the row to be updated.So if the caller has a Postgre SQL database, and calls with data to represent our third example (where the target values are all unique), then the Postgre SQL-specific subclass will effect the updates using the table / UPDATE … To match on names we now need to match on two columns.