Question

for instance if I have a table like this

total   date
10      2010-01-01
15      2010-01-02
98      2010-01-03
50      2010-01-05

how can I write select statement to get output like this?

total   date
10      2010-01-01
15      2010-01-02
98      2010-01-03
0       2010-01-04
50      2010-01-05

Answer 1

MySQL doesn’t have recursive functionality, so you’re left with using the NUMBERS table trick –

1. Create a table that only holds incrementing numbers – easy to do using an auto_increment:

   DROP TABLE IF EXISTS `example`.`numbers`;
   CREATE TABLE  `example`.`numbers` (
     `id` int(10) unsigned NOT NULL auto_increment,
      PRIMARY KEY  (`id`)
   ) ENGINE=InnoDB DEFAULT CHARSET=latin1;
   

2. Populate the table using:

   INSERT INTO NUMBERS
     (id)
   VALUES
     (NULL)
   

   …for as many values as you need. In this case, the INSERT statement needs to be run at least 31 times.

3. Use DATE_ADD to construct a list of days, increasing based on the NUMBERS.id value:

   SELECT x.dt
     FROM (SELECT DATE(DATE_ADD('2010-01-01', INTERVAL (n.id - 1) DAY)) AS dt
             FROM numbers n
            WHERE DATE_ADD('2010-01-01', INTERVAL (n.id - 1) DAY) <= '2010-01-05' ) x
   

4. Use an OUTER JOIN to get your desired output:

      SELECT x.dt,
             COUNT(*) AS total
        FROM (SELECT DATE(DATE_ADD('2010-01-01', INTERVAL (n.id - 1) DAY)) AS dt
                FROM numbers n
               WHERE DATE_ADD('2010-01-01', INTERVAL (n.id - 1) DAY) <= '2010-01-05' ) x
   LEFT JOIN YOUR_TABLE y ON y.date = x.dt
    GROUP BY x.dt
    ORDER BY x.dt
   

Why Numbers, not Dates?

Simple – dates can be generated based on the number, like in the example I provided. It also means using a single table, vs say one per data type.