I have developed a query, and in the results for the first three columns I get NULL. How can I replace it with 0?
select a,b,null,null from table1 union select null,null,c,d from table2 union select null,null,null,null,e,f
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..?
What is the difference between … and … and why does the latter not work?
Note: A NULL value is different from a zero value or a field that contains spaces.
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.
union all select null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null
Предикат IS NULL позволяет проверить отсутствие (наличие) значения в полях таблицы. Использование в этих случаях обычных предикатов сравнения может привести к неверным результатам, так как сравнение со значением NULL дает результат UNKNOWN (неизвестно).
In general, R supports: NULL NA NaN Inf / -Inf NULL is an object and is returned when an