100 totally free webcam usa - Updating multiple columns in mysql

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.

“value_columns” specifies the columns which will be updated (using SET). The second element provides the value to be SET in the column specified by “value_columns”.

In this case there are two distinct values, so we can do it in two UPDATE statements: So we can reduce the number of queries from five to two. But the extent to which this helps us reduces as the proportion of distinct SET values goes up.

Eventually every SET value is different, for example: UPDATE staff SET salary = 1125 WHERE name = ' Bob'; UPDATE staff SET salary = 1200 WHERE name = ' Jane'; UPDATE staff SET salary = 1100 WHERE name = ' Frank'; UPDATE staff SET salary = 1175 WHERE name = ' Susan'; UPDATE staff SET salary = 1150 WHERE name = ' John'; Since every SET clause is now different, we can’t coalesce any queries using the approaches mentioned before, so we still have one UPDATE statement for every row requiring changes. What if we had a database table which contained a representation of the updates we wanted to make, e.g.

The dominant factor in the time taken to complete the overall operation tends to be the “admin” work in conveying the application’s intention to the database server rather than the actual updates to the database.

A more effective solution to this problem is to attempt to reduce the number of UPDATE statements.

Let us start with a simple table: 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'; UPDATE staff SET salary = 1200 WHERE name = ' Bob'; UPDATE staff SET salary = 1250 WHERE name = ' Jane'; UPDATE staff SET salary = 1200 WHERE name = ' Frank'; UPDATE staff SET salary = 1250 WHERE name = ' Susan'; UPDATE staff SET salary = 1200 WHERE name = ' John'; We are no longer setting all the salary fields to the same value, so we can’t collapse it into a single statement.

But we can group the updates according to the value being set, and then do one UPDATE statement per distinct value.

In the case of Postgre SQL, it can be done using a FROM clause with UPDATE, like this: This does an INNER JOIN between the tables “staff” and “updates” where the column “name” matches.

The SET clause then takes the “salary” field from the “updates” table and uses it to update the “salary” field of the “staff” table.

A requirement arises in many systems to update multiple SQL database rows.

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