select a,b,null,null from table1 union select null,null,c,d from table2 union select null,null,null,null,e,f
I have developed a query, and in the results for the first three columns I get NULL. How can I replace it with 0?
What is the difference between … and … and why does the latter not work?
0x31303235343830303536 is NULL - they are just matching the number of columns in your existing query. If you had SELECT * FROM users and users had 4 columns, the UNION must also have 4 columns. As a result, they just used `NULL values to populate those columns.
Then in the second table I have select ..., null as opt from... I know that I could have an empty string with '' as opt however, I don't want an empty string, I really do need it to be null.
How can I run a MySQL query that selects everything that is not null? It would be something like … Do I just remove the all and go..?
Note: A NULL value is different from a zero value or a field that contains spaces.
union all select null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null
Даже выражение NULL != NULL не будет истинным, ведь нельзя однозначно сравнить одну неизвестность с другой. Кстати, ложным это выражение тоже не будет, потому что при вычислении условий Oracle не ограничивается состояниями ИСТИНА и ЛОЖЬ.
Null (or NULL) is a special marker used in Structured Query Language to indicate that a data value does not exist in the database. Introduced by the creator of the relational database model, E. F. Codd...