Question

首先是查询

SELECT * FROM mytable WHERE id='1'

给我一定数量的行例如

id             |       context      |      cat     |    value
1                     Context 1            1           value 1
1                     Context 2            1           value 2
1                     Context 1            2           value 3
1                     Context 2            2           value 4

现在我的问题不是以这种方式接收结果我想要的是这种方式

id      |   cat    |    Context 1     |  Context 2   
1            1           value 1          value 2       
1            2           value 3          value 4

Answer 1

@updated - 请参阅代码块3（MySQL）中的注释＃，该注释仅用于从控制台或HeidiSQL进行调试

您正在寻找的是旋转。您还没有说明这是什么DBMS，但对于SQL Server，请查看PIVOT运算符的联机丛书 - 如果类别是固定的，则可以使用。如果没有，那么你需要动态的SQL，比如

declare @sql nvarchar(max);

-- generate the column names
select @sql = coalesce(@sql + ',', '') + QuoteName(context)
from (select distinct context from mytable) T;

-- replace the column names into the generic PIVOT form
set @sql = REPLACE('
select id, cat, :columns:
from (select id, cat, context, value From mytable) p
pivot (max(value) for context in (:columns:)) pv',
':columns:', @sql)

-- execute for the results
exec (@sql)

测试数据

create table mytable (id int, context varchar(10), cat int, value varchar(20))
insert mytable select 1 ,'Context 1', 1 ,'value 1'
insert mytable select 1 ,'Context 2', 1 ,'value 2'
insert mytable select 1 ,'Context 1', 2 ,'value 3'
insert mytable select 1 ,'Context 2', 2 ,'value 4'

以上是针对SQL Server的。对于 MySQL ，请改用它，它遵循相同的技术。

SELECT @sql := NULL;
SELECT @sql := group_concat(
    'max(case when context=''',
    replace(context,'''',''''''),
    ''' then value end) `',
    replace(context,'`','``'),
    '`')
from (select distinct context from mytable) T;
SELECT @sql := concat(
    'select id, cat,',
    @sql,
    ' from mytable group by id, cat');
# SELECT @sql; # commented out, PHP sees this as the first result
PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

根据您的测试数据，加上两个curlies（引用和反引号）：

create table mytable (id int, context varchar(10), cat int, value varchar(20));
insert mytable select 1 ,'Context 1', 1 ,'value 1';
insert mytable select 1 ,'Context 2', 1 ,'value 2';
insert mytable select 1 ,'Context 1', 2 ,'value 3';
insert mytable select 1 ,'Context 2', 2 ,'value 4';
insert mytable select 1 ,'Context''3', 1 ,'quote';
insert mytable select 1 ,'Context`4', 2 ,'backquote';

Answer 2

SELECT  md.id, md.cat, m1.value AS context1, m2.value AS context2
FROM    (
        SELECT  DISTINCT id, cat
        FROM    mytable
        WHERE   id = 1
        ) md
LEFT JOIN
        mytable m1
ON      m1.id = 1
        AND m1.cat = md.cat
        AND m1.context = 'context 1'
LEFT JOIN
        mytable m2
ON      m2.id = 1
        AND m2.cat = md.cat
        AND m2.context = 'context 2'

Answer 3

SELECT t1.id, t1.cat, t1.value as 'Context 1', t2.value as 'Context 2'
FROM mytable t1, mytable t2
WHERE t1.id = t2.id AND t1.cat = t2.cat and t1.context = 'Context 1' and t2.context = 'Context 2'

如果每个ID / cat组合同时具有上下文1和上下文2，则

有效。

我们可以使用sql数据字段作为列名吗？

3 个答案: