Distinct Clause with Where

How to use DISTINCT in WHERE CLAUSE to avoid repeated row returns

You should apply group by clause.

SELECT 
baseFeeds.siteURL, feedFinishes.timeTaken, feedFinishes.timeSubmitted
FROM feedFinishes 
JOIN baseFeeds 
ON feedFinishes.GUID = baseFeeds.GUID 
WHERE baseFeeds.siteURL LIKE '%www.example.com%' 
GROUP BY
baseFeeds.siteURL, feedFinishes.timeTaken, feedFinishes.timeSubmitted

SQL: distinct in where clause

You don't state RDBMS. One way that should work in all.

SELECT Name,
       LasName,
       MAX(City) AS City
FROM   YourQuery
GROUP  BY Name,
          LasName

You say you don't care which city. This gives you the last alphabetically.

Or an alternative

SELECT Name,
       LasName,
       City
FROM   (SELECT Name,
               LasName,
               City,
               ROW_NUMBER() OVER (PARTITION BY Name, LasName ORDER BY (SELECT 0)) AS RN
        FROM   YourQuery) T
WHERE  RN = 1

Is there a way to ensure WHERE clause happens after DISTINCT?

You can do this without using a subquery using LEFT JOIN:

SELECT  c.id, c.comment_id_no, c.text, c.show, c.inserted_at
FROM    Comments AS c
        LEFT JOIN Comments AS c2
            ON c2.comment_id_no = c.comment_id_no
            AND c2.inserted_at > c.inserted_at
WHERE   c2.id IS NULL
AND     c.show = 'true';

I think all other approaches will require a subquery of some sort, this would usually be done with a ranking function:

SELECT  c.id, c.comment_id_no, c.text, c.show, c.inserted_at
FROM    (   SELECT  c.id, 
                    c.comment_id_no, 
                    c.text, 
                    c.show, 
                    c.inserted_at,
                    ROW_NUMBER() OVER(PARTITION BY c.comment_id_no 
                                      ORDER BY c.inserted_at DESC) AS RowNumber
            FROM    Comments AS c
        ) AS c
WHERE   c.RowNumber = 1
AND     c.show = 'true';

Since you have tagged with Postgresql you could also make use of DISTINCT ON ():

SELECT  *
FROM    (   SELECT  DISTINCT ON (c.comment_id_no) 
                    c.id, c.comment_id_no, c.text, c.show, c.inserted_at
            FROM    Comments AS c 
            ORDER By c.comment_id_no, inserted_at DESC
        ) x
WHERE   show = 'true';

Examples on DB<>Fiddle

SQL DISTINCT clause with WHERE

this should do this, depends on your DBMS

SELECT MAX(ID), Mail_ID FROM Subscriptions
    WHERE Klant_ID = '".$_GET["ID"]."' GROUP BY Mail_ID

then you have the right IDs, futher more you can get the rest information by

SELECT * FROM Subscriptions,
    (SELECT MAX(ID) as ids, Mail_ID FROM Subscriptions
        WHERE Klant_ID = '".$_GET["ID"]."' GROUP BY Mail_ID) table2
    WHERE ID=table2.ids

SQL select Distinct with where clause

Try this,

SELECT DISTINCT PersonID
FROM tableName
WHERE PersonID NOT IN
    (
        SELECT PersonID
        FROM tableName
        WHERE KvalifikationId = 2
    )

SQLFiddle Demo

SQL Distinct clause not working?

If you want the naming only appearing once, then group by comes to mind. One method is:

SELECT c.fldContactName,
       MAX(c.fldsignonlinesetup) as fldsignonlinesetup,
       MAX(c.fldorderdate) as fldorderdate,
       MAX(c.fldemail) as fldemail
FROM tblcustomers c LEFT JOIN
     tblorders o
     ON c.fldcustomerid = o.fldcustomerid
WHERE o.fldorderdate BETWEEN '2013-01-01' AND '2016-12-31' AND
      c.fldemail <> 'NULL' AND c.fldcontactname <> 'NULL' AND 
      c.fldcontactname <> '' AND c.fldemail <> '' AND
      c.fldsignonlinesetup = 0
GROUP BY c.fldcontactname
HAVING COUNT(*) = 1
ORDER BY c.fldcontactname ASC;

SELECT DISTINCT just makes sure that all the columns in the result set are never duplicates. It has nothing to do with finding values with only one row. The HAVING clause does this.

Notes:

The use of table aliases is good, but abbreviations for table names make the query more understandable.
The MAX() is really a no-op. With one row, it returns the value from the one row.
The GROUP BY is on the field you care about -- the one you don't want duplicates for.
The HAVING clause gets the values with only one row.
MySQL does not require the MAX() functions, but I strongly recommend using an aggregation function, so you don't learn bad habits that don't work in other databases and can behave unexpected in MySQL.
Do you really mean fldemail <> 'NULL' or do you intend A.fldemail IS NOT NULL?

How does Select query work with Distinct clause

SELECT DISTINCT is used to get set of unique rows. By other words, DISTINCT option cuts repeatable rows from a result. Most of times (most of db's actualy) you can't use distinct on not all but just some columns due to uncertainty of data.

Just for example, if your employees table looks like

Emp_id Dept_id Job_id
1      24      117
2      24      117
3      24      118
4      25      117

and you want to get list of unique jobs for every department, first you should try

SELECT Dept_id, Job_id
  FROM Employees

so result will be like:

Notice that now row "24 117" is doubled due to two employees with same job in one department. So, DISTINCT will cut duplicated rows in your result:

SELECT DISTINCT Dept_id, Job_id
  FROM Employees

gives:

24 117
24 118
25 117

But if you want DB to give you result of

SELECT Dept_id, DISTINCT Job_id

your DB won't be able to guess what exactly Dept_id you want to recieve in pair with 117 Job_id (choice is like "24 or 24 or 25?"). So if you want to get only distinct job_id's and some dept_id's along with them you should specify which exactly dept_id you want to see in you result if there will be many of them. For example you may get a minimum dept_id or something like that:

SELECT MIN(Dept_id) AS Dept_id, Job_id
FROM Employees
GROUP BY Job_id

with a result like:

24 117
24 118

It's all about certainty and uncertainty. In a sql query you should always specify what exactly you want.