计算数据库中多个字段等于不同值的行数

Question

Table books :

subject  |  userid  |  flag
---------+----------+--------
math     |  abc     |  0
math     |  abc     |  0
english  |  xyz     |  0

I wanna search the number of math subject where userid and flag = 0 .

How should I write the SQL statement?

I tried COUNT(subject) AS math FROM books WHERE userid = 'abc' AND flag = '0';

It doesn't work.

Answer 1

If you want to get only the math subjects with userid = 'abc' AND flag = '0' , you can use below query:

SELECT COUNT(subject) AS math 
FROM books WHERE userid = 'abc' AND flag = '0' AND subject = 'math';

check out the SQLFiddle

otherwise if you only want subjects with userid = 'abc' AND flag = '0' , the query given by you is perfectly doing that:

SELECT COUNT(subject) AS math 
FROM books WHERE userid = 'abc' AND flag = '0';

check out the SQLFiddle

For displaying in php using mysql_fetch_assoc you can give the column name to access the value like this:

$sql = "SELECT COUNT(subject) AS math 
    FROM books WHERE userid = 'abc' AND flag = '0'";

$result = mysql_query($sql);

if (!$result) {
    echo "Could not successfully run query ($sql) from DB: " . mysql_error();
    exit;
}

if (mysql_num_rows($result) == 0) {
    echo "No math subjects exist";
    exit;
}
while ($row = mysql_fetch_assoc($result)) {
    echo $row["math"];
}

Answer 2

Maybe this:

SELECT COUNT(subject) AS math
FROM books 
WHERE flag = '0'
GROUP BY userid
HAVING userid = 'abc';