I'm working with a little display complication here. I'm sure there's an IF/ELSE capability I'm just overlooking.
I have 2 tables I'm querying (customers, addresses). The first has the main record, but the second may or may not have a record to LEFT JOIN to.
I want to display a zero if there is no record in the addresses table. And I want to only display 1, if a record exists.
What I've attempted so far:
SELECT c.name, COALESCE(a.addressid,0) AS addressexists
FROM customers c
LEFT JOIN addresses a ON c.customerid = a.customerid
WHERE customerid = 123
This first example does not do it. But I may be utilizing COALESCE wrong.
How can I display a 0, if null, and a 1, if something exists?
Instead of COALESCE(a.addressid,0) AS addressexists
, use CASE
:
CASE WHEN a.addressid IS NOT NULL
THEN 1
ELSE 0
END AS addressexists
or the simpler:
(a.addressid IS NOT NULL) AS addressexists
This works because TRUE
is displayed as 1
in MySQL and FALSE
as 0
.
SELECT c.name, IF(a.addressid IS NULL,0,1) AS addressexists
FROM customers c
LEFT JOIN addresses a ON c.customerid = a.customerid
WHERE customerid = 123
Careful if you're coming from C/C++ and expecting this to work:
select if(name, 1, 0) ..
Even if 'name' is not NULL, unlike in C, a false-condition still triggers and the above statement returns 0. Thus, you have to remember to explicitly check for NULL or empty string:
select if(name is null or name = '', 0, 1)
PS Eugen's example up above is correct, but I wanted to clarify this nuance as it caught me by surprise.
SELECT
c.name,
CASE WHEN a.addressid IS NULL THEN 0 ELSE 1 END AS addressexists
FROM customers c
LEFT JOIN addresses a ON c.customerid = a.customerid
WHERE customerid = 123
Another method without WHERE, try this..
Will select both Empty and NULL values
SELECT ISNULL(NULLIF(fieldname,'')) FROM tablename
It will set null if it is an empty string, then be true on that also.
You can actually use an IF statement in the latest versions of MySQL.
IF(expr,if_true_expr,if_false_expr)
IE:
SELECT name, IF(ISNULL(name), 'robot', 'human') AS type
FROM visitors
If within TSQL, you can try :
SELECT IIF(a.addressid IS NULL, 0, 1) AS addressexists
SQL Server should work
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