Tabelle "user":
+==+=============+
+ID+ NAME        +
+==+=============+
+1 + John Doe    +
+2 + Test User   +
+==+=============+

Tabelle "texte":
+==+====+===============================+
+ID+USER+ TEXT                          +
+==+====+===============================+
+1 +  1 + Hallo du da!                  +
+2 +  1 + Wie geht es dir?              +
+3 +  2 + Mir gehts echt super.         +
+4 +  1 + Na das freut mich aber.       +
+==+====+===============================+

SELECT `id`, `name` FROM `user` LIMIT 10;

SELECT SUM(CHAR_LENGTH(`text`)) FROM `texte` WHERE `user` = 1;
SELECT SUM(CHAR_LENGTH(`text`)) FROM `texte` WHERE `user` = 2;
...

SELECT SUM(CHAR_LENGTH(`text`)) FROM `texte` WHERE `user` = 1;

select sum(text) from texte where user= 1

select u.id, u.name, t.id, t.text from user u
left outer join texte t
on u.id = t.user

select u.id, u.name, t.id, sum(t.text) from user u 
left outer join texte t 
on u.id = t.user
group by u.id, u.name, t.id

SELECT u.name, SUM(CHAR_LENGTH(t.text)) 
    FROM user u
    LEFT JOIN texte t ON u.id = t.user
    GROUP BY u.name

SELECT u.id, u.name, CHAR_LENGTH( t.text )
FROM user u
LEFT JOIN texte t ON u.id = t.user
GROUP BY u.name
ORDER BY u.id LIMIT 0 , 30