Question

I am trying to update rows in a data table that intersect rows in a smaller index table. The two tables are joined on the composite PK of the data table, and explain-select using the same criteria shows that the index is being used properly, and the correct unique rows are fetched – but I’m still having issues with the update.

The update on the joined tables works fine when there’s only 1 row in the temp table, but when I have more rows, I get MySql Error 1175, and none of the WHERE conditions I specify are recognized.

I’m aware that I can just switch off safe mode with SET SQL_SAFE_UPDATES=0, but can anyone tell me what I’m not understanding here? Why is my WHERE condition not accepted, and why does it even need a where when I’m doing a NATURAL JOIN – and why does this work with only one row in the right-hand-side table (MyTempTable)?

The Code

Below is vastly simplified, but structurally identical create table & updates representing my problem.

-- The Data Table.
Create Table MyDataTable
(
    KeyPartOne int not null,
    KeyPartTwo varchar(64) not null,
    KeyPartThree int not null,
    RelevantData varchar(200) null,
    Primary key (KeyPartOne, KeyPartTwo, KeyPartThree)
) Engine=InnoDB;

-- The 'Temp' table.
Create Table MyTempTable
(
    KeyPartOne int not null,
    KeyPartTwo varchar(64) not null,
    KeyPartThree int not null,
    Primary key (KeyPartOne, KeyPartTwo, KeyPartThree)
)Engine=Memory;

-- The Update Query (works fine with only 1 row in Temp table)
update MyDataTable natural join MyTempTable 
set RelevantData = 'Something Meaningful';

-- Specifying 'where' - roduces same effect as the other update query
update MyDataTable mdt join MyTempTable mtt
on mdt.KeyPartOne = mtt.KeyPartOne
and mdt.KeyPartTwo = mtt.KeyPartTwo
and mdt.KeyPartThree = mtt.KeyPartThree
set RelevantData = 'Something Meaningful'
where mdt.KeyPartOne = mtt.KeyPartOne
and mdt.KeyPartTwo = mtt.KeyPartTwo
and mdt.KeyPartThree = mtt.KeyPartThree;

P.S. Both of the above update statements work as expected when the temp table contains only one row, but give me the error when there’s more than one row. I’m seriously curious about why!

Answer 1

Per the MySql forum, the update queries are valid, and the fact that they don’t work in Workbench with safe-update mode turned on does not indicate that there’s anything wrong with the index. It’s just a quirk of Workbench’s “don’t-shoot-yourself-in-the-foot” mode. 🙂