In addition to adding new data, a common task is to update existing data, which can be done using an UPDATE statement. Similar to the INSERT statement, you have to specify exactly which table, columns, and rows to update. In addition, the data you are updating has to match the data type of the columns in the table schema.

--Update statement with values
UPDATE mytable
SET column = value_or_expr, 
    other_column = another_value_or_expr, 
    …
WHERE condition;

The statement works by taking multiple column/value pairs, and applying those changes to each and every row that satisfies the constraint in the WHERE clause.

Taking care

Most people working with SQL will make mistakes updating data at one point or another. Whether it's updating the wrong set of rows in a production database, or accidentally leaving out the WHERE clause (which causes the update to apply to all rows), you need to be extra careful when constructing UPDATE statements.

One helpful tip is to always write the constraint first and test it in a SELECT query to make sure you are updating the right rows, and only then writing the column/value pairs to update.