postgresql – 左外连接 – 如何在第二个表中返回存在的布尔值？

在CentOS 6上的Postgresql 9中,pref_users表中有60000条记录：

# \d pref_users
                   Table "public.pref_users"
   Column   |            Type             |     Modifiers      
------------+-----------------------------+--------------------
 id         | character varying(32)       | not null
 first_name | character varying(64)       | not null
 last_name  | character varying(64)       | 
 login      | timestamp without time zone | default Now()
 last_ip    | inet                        | 
 (... more columns skipped...)

另一个表格中包含大约500个用户不允许再玩的内容：

# \d pref_ban2
                 Table "public.pref_ban2"
   Column   |            Type             |   Modifiers   
------------+-----------------------------+---------------
 id         | character varying(32)       | not null
 first_name | character varying(64)       | 
 last_name  | character varying(64)       | 
 city       | character varying(64)       | 
 last_ip    | inet                        | 
 reason     | character varying(128)      | 
 created    | timestamp without time zone | default Now()
Indexes:
    "pref_ban2_pkey" PRIMARY KEY,btree (id)

在PHP脚本中,我试图在jQuery-dataTable中显示来自pref_users的所有60000个用户.我想标记被禁用的用户(在pref_ban2中找到的用户).

这意味着我需要一个名为ban的列,用于查询中包含true或false的每个记录.

所以我正在尝试左外连接查询：

# select                          
       b.id,-- how to make this column a boolean?
       u.id,u.first_name,u.last_name,u.city,u.last_ip,to_char(u.login,'DD.MM.YYYY') as day
from pref_users u left outer join pref_ban2 b on u.id=b.id
limit 10;
 id |    id    | first_name  | last_name |    city     |     last_ip     |    day     
----+----------+-------------+-----------+-------------+-----------------+------------
    | DE1      | Alex        |           | Bochum      | 2.206.0.224     | 21.11.2014
    | DE100032 | Княжна Мэри |           | London      | 151.50.61.131   | 01.02.2014
    | DE10011  | Aлександр Ш |           | Симферополь | 37.57.108.13    | 01.01.2014
    | DE10016  | Semen10     |           | usa         | 69.123.171.15   | 25.06.2014
    | DE10018  | Горловка    |           | Горловка    | 178.216.97.214  | 25.09.2011
    | DE10019  | -Дмитрий-   |           | пермь       | 5.140.81.95     | 21.11.2014
    | DE10047  | Василий     |           | Cумы        | 95.132.42.185   | 25.07.2014
    | DE10054  | Maedhros    |           | Чикаго      | 207.246.176.110 | 26.06.2014
    | DE10062  | ssergw      |           | москва      | 46.188.125.206  | 12.09.2014
    | DE10086  | Вадим       |           | Тула        | 109.111.26.176  | 26.02.2012
(10 rows)

如您所见,上面的b.id列为空 – 因为这10个用户未被禁止.

如何在该列中获取false值而不是String？

而且我不是在一些coalesceor case表达式之后,但我正在寻找“正确”的方式来进行这样的查询.

具有外连接的CASE或COALESCE语句是执行此操作的正确方法.

select
  CASE 
    WHEN b.id IS NULL THEN true
    ELSE false
  END AS banned,u.id,'DD.MM.YYYY') as day
from pref_users u 
left outer join pref_ban2 b 
  on u.id=b.id
limit 10;