如何解决mysql中“子查询返回超过1行”的错误？

我在codeigniter中提出了如下查询

$query = $this->db->query('SELECT tanggal,
            ( SELECT COUNT(*) from absensi_siswa where kehadiran="alfa" GROUP BY tanggal ) AS alfa,
            ( SELECT COUNT(*) from absensi_siswa where kehadiran="izin" GROUP BY tanggal ) AS izin,
            ( SELECT COUNT(*) from absensi_siswa where kehadiran="hadir" GROUP BY tanggal ) AS hadir,
            ( SELECT COUNT(*) from absensi_siswa where kehadiran="sakit" GROUP BY tanggal ) AS sakit
            FROM absensi_siswa GROUP BY tanggal');
    return $query->result();

您的每个子条款选择count*。。返回超过1行，从您的查询中，我假设您需要基于kehadiran的不同值的单个计数，您可以将查询简化为

SELECT tanggal,
SUM(CASE WHEN kehadiran="alfa" THEN 1 ELSE 0 END) AS alfa,
SUM(CASE WHEN kehadiran="izin" THEN 1 ELSE 0 END) AS izin,
SUM(CASE WHEN kehadiran="hadir" THEN 1 ELSE 0 END) AS hadir,
SUM(CASE WHEN kehadiran="sakit" THEN 1 ELSE 0 END) AS sakit
FROM absensi_siswa 
GROUP BY tanggal

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使用mysql，您可以使sum子句更短，比如将sumkehadiran=alfa作为alfa，将表达式放在suma=a中将返回布尔值0/1或true/false，您将得到基于表达式的计数。祝您有个美好的一天。