What is the best way to convert an int or null to boolean value in an SQL query?

问题

What is the best way to convert an int or null to boolean value in an SQL query, such that:

Any non-null value is TRUE in the results
Any null value is FALSE in the results

回答1:

To my knowledge (correct me if I'm wrong), there is no concept of literal boolean values in SQL. You can have expressions evaluating to boolean values, but you cannot output them.

This said, you can use CASE WHEN to produce a value you can use in a comparison:

SELECT 
    CASE WHEN ValueColumn IS NULL THEN 'FALSE' ELSE 'TRUE' END BooleanOutput 
FROM 
    table

回答2:

No need to use case... when:

select (column_name is not null) as result from table_name;

Returns 1 for all fields not NULL and 0 for all fields that are NULL, which is as close as you can get to booleans in SQL.

回答3:

SELECT 
    CASE
        WHEN thevalue IS NULL THEN 0
        ELSE 1
    END AS newVal
FROM .... (rest of select)

I think it goes something like this

Actually, the ISNULL, may need to be WHEN thevalue IS NULL THEN 0

回答4:

Assuming you want 0,1 value as a return, and that we are talking about integer I would use the logic specified by Torbjörn and wrap it in the function

create function dbo.isNotNull(@a int)
returns bit
as
begin
return isnull(@a-@a+1,0)
end

so then you can use it whenever you need by simply calling

select dbo.isNotNull(myIntColumn) from myTable

The answer provided by Tomalak is more universal though as it would work with any data type

回答5:

You may want to do a Convert(BIT, Value) of your result. Because something SQL will return an error that the value is not a boolean.

回答6:

isnull(column - column + 1, 0) != 0

回答7:

In Oracle, assuming you use 0 for false and 1 for true:-

SELECT DECODE( col, NULL, 0, 1) FROM ...

You can also write this using the CASE syntax, but the above is idiomatic in Oracle. DECODE is a bit like a switch/case; if col is NULL then 0 is returned, else 1.

回答8:

The shortest one I know for Oracle:

SELECT NVL2(nullableColumn, 1, 0) FROM someTable

NVL2(value, ifNotNull, ifNull) returns ifNotNull if the value is not null, and ifNull otherwise.

回答9:

Usually when use 1, it means is true and else in other case.

So:

SELECT  IsDefault = CASE WHEN IsDefault = 1 THEN 'true' ELSE 'false' END
FROM table

回答10:

The syntax works, but I had to figure out how to place it in my query. If OK, I'd like to share an example on how to fit into an extended query:

select count(*) as count, inventory,
CASE WHEN inventory = 0 THEN 'empty' ELSE 'not empty' END as InventoryStatus
from mytable group by count, inventory

来源：https://stackoverflow.com/questions/177956/what-is-the-best-way-to-convert-an-int-or-null-to-boolean-value-in-an-sql-query

标签

sql

null

boolean

type-conversion