How it is? Did the Update command Truncate the tables?

Question

MySQL

20 Update requests, I wanted to optimize them. To merge these 20 requests into one, the WHEN - THEN construct was used. Late, fatigue, mistake.
As a result of the query, all values ​​in the brand_sku_increment table disappeared!
Yes, everything was restored, but I can not understand how this could happen. When there is a syntax error, an error warning is displayed and the command simply fails.

UPDATE brand_sku_increment SET `sku_brand`= CASE
WHEN product_sku = 'G-60019 THEN '00006-1'
WHEN product_sku = 'G-40015' THEN '00001-1'
......

END 
WHERE 1

Help to understand what caused truncate.
product_sku and brand_sku - primary key Ca
n't they be changed from inside the table at all?

Answer 1

It is this CASE WHEN that bothers me more. After all, you only have THEN, but you don't have ELSE. In theory, if ELSE is omitted, then you need to insert a NULL value. And also the WHERE 1 condition is alarming