Check out example codes for "mysql update add to existing value". It will help you in understanding the concepts better.

Code Example 1

mysql> UPDATE tableName SET sometotal = sometotal+10 WHERE Id = 4;

Code Example 2

UPDATE [LOW_PRIORITY] [IGNORE] table_name 
SET 
    column_name1 = expr1,
    column_name2 = expr2,
    ...
[WHERE
    condition];

Code Example 3

-- Things in brackets are optional
-- IGNORE modifier updates rows even if errors occur (ie: the rows that cause errors are simply not updated)
UPDATE [IGNORE] table_name 
SET 
    column_name1 = expr1,
    column_name2 = expr2,
    ...
[WHERE 
    condition]; -- WHERE tells us which rows to update based on said condition

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