# Question

Why does query 1 not return the same items as query 2?  Effectively, it works the same as query 4.

## background

MySQL Server version: 5.1.73-log Source distribution

mysql client Ver 14.14 Distrib 5.7.22, for Linux (x86_64) using  EditLine wrapper

### schema
```
mysql> desc eprint;
+---------------------------+---------------------+------+-----+---------+-------+
| Field                     | Type                | Null | Key | Default | Extra |
+---------------------------+---------------------+------+-----+---------+-------+
| eprintid                  | int(11)             | NO   | PRI | NULL    |       |
...
| abstract_nicht_anzeigen   | set('TRUE','FALSE') | YES  |     | NULL    |       |
```

### value distribution
```
mysql> select count(*),abstract_nicht_anzeigen from eprint group by abstract_nicht_anzeigen;                                                             +----------+-------------------------+
| count(*) | abstract_nicht_anzeigen |
+----------+-------------------------+
|    19766 | NULL                    |
|    32177 |                         |
|      736 | TRUE                    |
+----------+-------------------------+
3 rows in set (0.23 sec)
```

## query 1

```
SELECT eprintid,ac_nummer,urn,thesis_type
  FROM eprint
 WHERE eprint_status="archive" and metadata_visibility="show" and date_sperre_year is null
       and einverstaendnis=TRUE and full_text_status="public"
       and abstract_nicht_anzeigen<>TRUE and urn is not null;
...
19544 rows in set (0.29 sec)
```

## query 2
```
SELECT eprintid,ac_nummer,urn,thesis_type
  FROM eprint
 WHERE eprint_status="archive" and metadata_visibility="show" and date_sperre_year is null
       and einverstaendnis=TRUE and full_text_status="public"
       and (abstract_nicht_anzeigen is NULL or abstract_nicht_anzeigen="") and urn is not null;
...
32651 rows in set (0.02 sec)
```

## query 3
```
SELECT eprintid,ac_nummer,urn,thesis_type
  FROM eprint
 WHERE eprint_status="archive" and metadata_visibility="show" and date_sperre_year is null
       and einverstaendnis=TRUE and full_text_status="public"
       and abstract_nicht_anzeigen is NULL and urn is not null;
...
13107 rows in set (0.30 sec)
```

## query 4
```
SELECT eprintid,ac_nummer,urn,thesis_type
  FROM eprint
 WHERE eprint_status="archive" and metadata_visibility="show" and date_sperre_year is null
       and einverstaendnis=TRUE and full_text_status="public"
       and abstract_nicht_anzeigen="" and urn is not null;
...
19544 rows in set (0.29 sec)
```