我不能谈论 MySQL,但在 PostgreSQL 中你可以使用 crosstab 函数表函数 http://www.postgresql.org/docs/current/static/tablefunc.html module:
CREATE OR REPLACE VIEW PersonAttributePivot AS
SELECT PersonId AS ID, Age, HairColor
FROM crosstab
(
'SELECT PersonId, Key, Value FROM PersonAttribute',
'SELECT DISTINCT Key FROM PersonAttribute ORDER BY Key'
)
AS
(
PersonId integer,
Age text,
HairColor text
);
加入查询:
SELECT id, name, age, haircolor
FROM Person JOIN PersonAttributePivot USING(id)
ORDER BY id;
想要的结果:
id | name | age | haircolor
----+------------+-----+-----------
1 | Joe Bloggs | 27 |
2 | Jane Doe | | Brown
(2 rows)
正如你所看到的,我将明确的列列表放入PersonAttributePivot
看法。我不知道任何带有隐式列列表的“自动透视”创建方式。
EDIT:
For huge列列表(假设总是text
type)作为一种解决方法,我看到了这样一点点修改的方法:
动态类型创建(这里简单地基于 Java):
Class.forName("org.postgresql.Driver");
Connection c =
DriverManager.getConnection("jdbc:postgresql://localhost/postgres", "postgres", "12345");
Statement s = c.createStatement();
ResultSet rs = s.executeQuery("SELECT DISTINCT Key FROM PersonAttribute ORDER BY Key");
List<String> columns = new ArrayList<String>();
while (rs.next())
columns.add(rs.getString(1));
System.out.println("CREATE TYPE PersonAttributePivotType AS (");
System.out.println("\tPersonId integer,");
for (int i = 0; i < columns.size(); ++i)
{
System.out.print("\t" + columns.get(i) + " text");
if (i != columns.size() - 1)
System.out.print(",");
System.out.println();
}
System.out.println(");");
Result:
CREATE TYPE PersonAttributePivotType AS (
PersonId integer,
Age text,
HairColor text
);
函数包装器:
CREATE OR REPLACE FUNCTION crosstabPersonAttribute(text, text)
RETURNS setof PersonAttributePivotType
AS '$libdir/tablefunc','crosstab_hash' LANGUAGE C STABLE STRICT;
自动视图创建:
CREATE OR REPLACE VIEW PersonAttributePivot AS
SELECT * FROM crosstabPersonAttribute
(
'SELECT PersonId, Key, Value FROM PersonAttribute',
'SELECT DISTINCT Key FROM PersonAttribute ORDER BY Key'
);
Result:
TABLE PersonAttributePivot;
personid | age | haircolor
----------+-----+-----------
1 | 27 |
2 | | Brown
(2 rows)