Question

I have one table in MySQL database (just example)

CREATE  TABLE test.Names (
    id INT NOT NULL ,  
    salary INT NULL ,  
    name VARCHAR(45) NULL , 
    tmpl INT,
    PRIMARY KEY (`id`) ,  
    UNIQUE INDEX `id_UNIQUE` (`id` ASC)
);

I have these records

id,   salary, name, tmpl
1,    10,     John, null 
2,    20,     Pat,  null 
3,    30,     Jane, null

I’d like update the table by this simple calculation for each person

salary of each person/Sum(salary)*100

So the result should be

id,   salary, name, tmpl
1,    10,     John, 16.67 
2,    20,     Pat,  33.33 
3,    30,     Jane, 50

How to perform this by one SQL query? Or what is the best approach (I assume the real table will have >100 000 rows)

Answer 1

Use a separate SELECT statement to get the total salary – either put that in a variable, or cross-join this with your existing table. Then you have the sum available for each individual record.

This SQL code suffices:

SET @sum_salaries = (SELECT SUM(salary) FROM test.Names);

UPDATE test.Names
SET tmpl = salary/@sum_salaries * 100;

No need for a join, even. Simpler this way and probably faster.