如果列为空，则mysql select语句将返回列

Question

有没有办法select col1 if col2 is empty else use col2？ col1永远不会是空的。

$sql = 'select 
        ug.permissions,
        g.name,
        g.display,
        g.permissions
        from system.users_groups as ug
        inner join system.groups as g on ug.group_id = g.id
        where ug.user_id = ' . $_SESSION['user_id'];

上面的语句带回了两个权限列，我想只返回一个权限列。

if ug.permissions = '' then use g.permissions

Answer 1

使用COALESCE和NULLIF：

$sql = 'select 
        COALESCE(NULLIF(ug.permissions,''),g.permissions) as permissions ,
        g.name,
        g.display
        from system.users_groups as ug
        inner join system.groups as g on ug.group_id = g.id
        where ug.user_id = ' . $_SESSION['user_id'];

使用CASE：

$sql = 'select 
        CASE WHEN ug.permissions = '' OR ug.permissions IS NULL
        THEN
             g.permissions
        ELSE
             ug.permissions 
        END as permissions ,
        g.name,
        g.display
        from system.users_groups as ug
        inner join system.groups as g on ug.group_id = g.id
        where ug.user_id = ' . $_SESSION['user_id'];