COALESCE和IFNULL不能按预期工作

Question

I'm trying to output a default row when there's no row found in the query. This is a sample of my query:

SELECT 
COALESCE(site, 'STE') as site, 
instrument,
field

FROM Table1
WHERE site IN ('East', 'West')
AND DATE(tstamp) = "2016-09-07"
ORDER BY id desc

The output is

+------+------------+-------+
| site | instrument | field |
+------+------------+-------+
| West | 0          | 0     |
+------+------------+-------+

For the tsamp 2016-09-07 we have a row for the site "West" and there's no row for "East". I tried to search and found that I can use COALESCE and also tried IFNULL but I'm only getting the output above. I also tried if(count(site) = 0, "STE", site) but i can't get it to work.

My expected result is

+------+------------+-------+
| site | instrument | field |
+------+------------+-------+
| West | 0          | 0     |
| STE  | NULL       | NULL  |
+------+------------+-------+   

I hope you guys can help me. Thanks in advance

Answer 1

Both coalesce() and ifnull() work on a row basis, meaning they can replace a null value if that null value exists in a record. However, they cannot create a record that does not exist - and you do not have any records matching East (or STE).

A possible solution is to create a table that has all possible values for the site field and you can left join on this table:

SELECT 
COALESCE(Table1.site, "STE") as site, 
Table1.instrument,
Table1.field

FROM LookupTable lt
LEFT JOIN Table1 ON lt.site=Table1.site
WHERE lt.site IN ('East', 'West')
AND DATE(Table1.tstamp) = "2016-09-07"
ORDER BY id desc

Answer 2

If 'STE' isn't in table1.site then it won't come back in the results.

You could do a union instead:

SELECT 
site, 
instrument,
field

FROM (SELECT 'West' [Site], 0 [instrument], 0 [field]) table1
WHERE site IN ('East', 'West')
UNION 
SELECT 'STE', NULL, NULL

Note that by using "STE" you're looking for the column name "STE", not the value 'STE' (single quotes)

EDIT

You need a control table to dictate which values to look for, or you have to hard code your rules. You can't look for something that's missing without first specifying the things that should be there.

Here's an option:

--Create and populate reference table "sites"
create table sites
    ([site] char (4))

INSERT INTO sites VALUES ('East')
INSERT INTO sites VALUES ('West')

-- Query against reference table
SELECT 
ISNULL(table1.site, 'STE'), 
instrument,
field
FROM (SELECT 'West' [Site], 0 [instrument], 0 [field]) table1
RIGHT JOIN sites on table1.[Site] = sites.[Site]

--or

-- Query against reference table
SELECT 
ISNULL(table1.site, 'STE'), 
instrument,
field
FROM sites
LEFT JOIN (SELECT 'West' [Site], 0 [instrument], 0 [field]) table1  on table1.[Site] = sites.[Site]

Let me know if you have questions on how this works.