Mysql 'or' operand is broken?

See more:

MySQL

I posted this on Stack but got no traffic:

I have 2 web apps (aspx/c#) set up on two databases. The software is identical.

One database is 5.6, the other has just been upgraded to 5.7, which is when this issue arose

I have an entity framework model of the mysql databases. The query is as follows:

db.ldz_from_rec.GroupBy(l => l.region, (key, g) => g.OrderBy(l=>l.LDZ_Exit).FirstOrDefault());

the result from view 'ldz_from_rec' has about 14 unique 'regions' but loads of LDZs per region. I just want one.

Here is the sql EF spits at mysql:

SELECT
`Apply1`.`id`, 
`Apply1`.`REGION1` AS `region`, 
`Apply1`.`LDZ_Id`, 
`Apply1`.`LDZ_Exit`
FROM (SELECT
`Distinct1`.`region`, 
(SELECT
`Project2`.`id`
FROM `ldz_from_rec` AS `Project2`
 WHERE (`Distinct1`.`region` = `Project2`.`region`) OR ((`Distinct1`.`region` IS  NULL) AND (`Project2`.`region` IS  NULL))
 ORDER BY 
`Project2`.`LDZ_Exit` ASC LIMIT 1) AS `id`, 
(SELECT
`Project2`.`region`
FROM `ldz_from_rec` AS `Project2`
 WHERE (`Distinct1`.`region` = `Project2`.`region`) OR ((`Distinct1`.`region` IS  NULL) AND (`Project2`.`region` IS  NULL))
 ORDER BY 
`Project2`.`LDZ_Exit` ASC LIMIT 1) AS `REGION1`, 
(SELECT
`Project2`.`LDZ_Id`
FROM `ldz_from_rec` AS `Project2`
 WHERE (`Distinct1`.`region` = `Project2`.`region`) OR ((`Distinct1`.`region` IS  NULL) AND (`Project2`.`region` IS  NULL))
 ORDER BY 
`Project2`.`LDZ_Exit` ASC LIMIT 1) AS `LDZ_Id`, 
(SELECT
`Project2`.`LDZ_Exit`
FROM `ldz_from_rec` AS `Project2`
 WHERE (`Distinct1`.`region` = `Project2`.`region`) OR ((`Distinct1`.`region` IS  NULL) AND (`Project2`.`region` IS  NULL))
 ORDER BY 
`Project2`.`LDZ_Exit` ASC LIMIT 1) AS `LDZ_Exit`
FROM (SELECT DISTINCT 
`Extent1`.`region`
FROM `ldz_from_rec` AS `Extent1`) AS `Distinct1`) AS `Apply1`

On one (5.6) database it works fine, on the other (5.7) returns null for all columns, but it does select 14 rows of nulls.

I tested the query on the databases to confirm what EF was getting. I found that in the db that didn't work, I could change the OR to and XOR and it works.

See my example:

select *,
(SELECT     `Project2`.`id`     
FROM `ldz_from_rec` AS `Project2`
/* the next line returns results */
 WHERE (`Distinct1`.`region` = `Project2`.`region`) XOR ((`Distinct1`.`region` IS  NULL) AND (`Project2`.`region` IS  NULL))
/* the next line does not return results */
 /*WHERE (`Distinct1`.`region` = `Project2`.`region`) OR ((`Distinct1`.`region` IS  NULL) AND (`Project2`.`region` IS  NULL))*/
 ORDER BY 
`Project2`.`LDZ_Exit` ASC LIMIT 1) AS `id`
from (SELECT DISTINCT 
`Extent1`.`region`
FROM `ldz_from_rec` AS `Extent1`) as `Distinct1` ;

I get that the rows are null because no rows were returned in the subqueries but how then does XOR find rows that OR could not. I cannot think of any examples where XOR can possibly return more that OR :S

This does not happen in all cases of using OR, but it is consistent in this example

Am I going mad?
Is MySql Broken?
How can I fix this?
Can I frame my lamda better to avoid this?

Thanks ^_^ Andy

What I have tried:

select *,
(SELECT     `Project2`.`id`     
FROM `ldz_from_rec` AS `Project2`
/* the next line returns results */
 WHERE (`Distinct1`.`region` = `Project2`.`region`) XOR ((`Distinct1`.`region` IS  NULL) AND (`Project2`.`region` IS  NULL))
/* the next line does not return results */
 /*WHERE (`Distinct1`.`region` = `Project2`.`region`) OR ((`Distinct1`.`region` IS  NULL) AND (`Project2`.`region` IS  NULL))*/
 ORDER BY 
`Project2`.`LDZ_Exit` ASC LIMIT 1) AS `id`
from (SELECT DISTINCT 
`Extent1`.`region`
FROM `ldz_from_rec` AS `Extent1`) as `Distinct1` ;