计算具有相同ID的行数[PHP]

Question

So I want to count and echo the number of rows from table2 , which have the same id row as table1 .

I understand the function mysqli_num_rows funtion, but how can I make it count the rows from table2 that have the matching id ?

Total newbie, and still searching. Thought this might help me get in the right direction. Thanks for any help !

Answer 1

mysqli_num_rows return the number of rows from a result. You should write an other query to return the number of rows in table 2 with the same id

SELECT COUNT(*)
FROM table2
WHERE table2.id = <id>

Answer 2

CREATE TABLE table1 (id INT);
CREATE TABLE table2 (id INT);

INSERT INTO table1 VALUES (1), (2), (3), (4);
INSERT INTO table2 VALUES (2), (4);

SELECT COUNT(*)
  FROM table1
  JOIN table2
    ON table1.id = table2.id;

Try it here: http://sqlfiddle.com/#!9/0d289/1

Answer 3

Try this:

    SELECT `table2`.`id`, COUNT(`table2`.`id`) AS `num_rows`
    FROM `table2`
    LEFT JOIN `table1` ON `table2`.`id` = `table1`.`id`
    WHERE `table2`.`id` IS NOT NULL
    GROUP BY `table2`.`id`
    ORDER BY `table2`.`id` ASC

Answer 4

To only count

select count(*)
  from table1, table2
  where  table1.id = table2.id

To know what id and how much

select table2.id, count(*)
  from table1, table2
    where  table1.id = table2.id
  group by table2.id 

The second case on sqlfiddle.com