decode函数和行列互换

来源：互联网发布：软件特性是哪些编辑：程序博客网时间：2024/04/28 11:41

Decode函数的语法结构如下:

    decode (expression, search_1, result_1)
    decode (expression, search_1, result_1, search_2, result_2)
    decode (expression, search_1, result_1, search_2, result_2, ....,
    search_n, result_n)
    decode (expression, search_1, result_1, default)
    decode (expression, search_1, result_1, search_2, result_2, default)
    decode (expression, search_1, result_1, search_2, result_2, ....,
    search_n, result_n, default)

    作用：和case表达式实现的功能一样，以第1 个参数为关键字段，如果等于search，则结果为对应的result，最后匹配不到的结果为default。如果没有写default，返回null。为了格式一致，建议匹配不到不返回结果时，写null。如果第一个返回结果为char 或null，oracle自动转型为varchar2，如下面例子

    SELECT product_id,
    DECODE (warehouse_id, 1, ’Southlake’,
    2, ’San Francisco’,
    3, ’New Jersey’,
    4, ’Seattle’,
    ’Non-domestic’)
    "Location of inventory" FROM inventories
    WHERE product_id < 1775;

    Decode 函数的主要使用:

    求一个中文排序方法，谢谢！

    SELECT * FROM TEMP ORDER BY 发送状态;

    发送状态东城区西城区崇文区宣武区朝阳区丰台区石景山区海淀区合计
    成功         1,473    1,113     297      103       875       1,037         7            576    5,481
    发送中      144          3          14          0          30            0            11              8         210
    合计         1,619    1,116     324      103      1,125     1,224       47            584    6,142
    失败             2           0           13          0          220        187         29              0          451

    想按照如下顺序排序，指定按照“发送状态”字段“成功”-“发送中”，“失败”,"合计"的顺序，即：

    发送状态东城区西城区崇文区宣武区朝阳区丰台区石景山区海淀区合计
    成功         1,473    1,113     297      103       875       1,037         7            576    5,481
    发送中      144          3          14          0          30            0            11              8         210
    失败             2           0           13          0          220        187         29              0          451
    合计         1,619    1,116     324      103      1,125     1,224       47            584     6,142

    SELECT * FROM TEMP ORDER BY DECODE(发送状态,'成功',1,'发送中',2,....)

    来源于select name,id from tmp order by 2;按列位置排序，使用decode则可以动态指定

    主要使用在查询结果的行列转换上，如：

    1、固定列数的行列转换

    如

    name subject grade
    ---------------------------
    student1 语文 80
    student1 数学 70
    student1 英语 60
    student2 语文 90
    student2 数学 80
    student2 英语 100
    ……

    转换为

    语文数学英语
    student1 80 70 60
    student2 90 80 100

    按照name分组得到的结果，所以decode外面要套个sum，不然group by出现语义错误，结果为

    select name,sum(decode(subject,'语文', grade,null)) "语文",
    sum(decode(subject,'数学', grade,null)) "数学",
    sum(decode(subject,'英语', grade,null)) "英语"
    from student
    group by name

    2、不定列行列转换

    如

    c1 c2
    --------------
    1 我
    1 是
    1 谁
    2 知
    2 道
    3 不
    ……

    转换为

    1 我是谁
    2 知道
    3 不

    这一类型的转换必须借助于PL/SQL 来完成，这里给一个例子

    CREATE OR REPLACE FUNCTION get_c2(tmp_c1 NUMBER)
    RETURN VARCHAR2
    IS
    Col_c2 VARCHAR2(4000);
    BEGIN
    FOR cur IN (SELECT c2 FROM t WHERE c1=tmp_c1) LOOP
    Col_c2 := Col_c2||cur.c2;
    END LOOP;
    Col_c2 := rtrim(Col_c2,1);
    RETURN Col_c2;
    END;
    /
    SQL> select distinct c1 ,get_c2(c1) cc2 from table;即可

    或者不用pl/sql，利用分析函数和 CONNECT_BY 实现：

    SELECT c1, SUBSTR (MAX (SYS_CONNECT_BY_PATH (c2, ';')), 2) NAME
    FROM (SELECT c1, c2, rn, LEAD (rn) OVER (PARTITION BY c1 ORDER BY rn) rn1
    FROM (SELECT c1, c2, ROW_NUMBER () OVER (ORDER BY c2) rn
    FROM t))
    START WITH rn1 IS NULL
    CONNECT BY rn1 = PRIOR rn
    GROUP BY c1;

    3、列数不固定（交叉表行列转置）

    这种是比较麻烦的一种，需要借助pl/sql：

    原始数据：

    CLASS1 CALLDATE CALLCOUNT
    1 2005-08-08 40
    1 2005-08-07 6
    2 2005-08-08 77
    3 2005-08-09 33
    3 2005-08-08 9
    3 2005-08-07 21

    转置后：

    CALLDATE CallCount1 CallCount2 CallCount3
    ------------ ---------- ---------- ----------
    2005-08-09 0 0 33
    2005-08-08 40 77 9
    2005-08-07 6 0 21

    试验如下：

    1）. 建立测试表和数据

    CREATE TABLE t(
    class1 VARCHAR2(2 BYTE),
    calldate DATE,
    callcount INTEGER
    );
    INSERT INTO t(class1, calldate, callcount)
    VALUES ('1', TO_DATE ('08/08/2005', 'MM/DD/YYYY'), 40);
    INSERT INTO t(class1, calldate, callcount)
    VALUES ('1', TO_DATE ('08/07/2005', 'MM/DD/YYYY'), 6);
    INSERT INTO t(class1, calldate, callcount)
    VALUES ('2', TO_DATE ('08/08/2005', 'MM/DD/YYYY'), 77);
    INSERT INTO t(class1, calldate, callcount)
    VALUES ('3', TO_DATE ('08/09/2005', 'MM/DD/YYYY'), 33);
    INSERT INTO t(class1, calldate, callcount)
    VALUES ('3', TO_DATE ('08/08/2005', 'MM/DD/YYYY'), 9);
    INSERT INTO t(class1, calldate, callcount)
    VALUES ('3', TO_DATE ('08/07/2005', 'MM/DD/YYYY'), 21);
    COMMIT ;

    2）建立ref cursor准备输出结果集

    CREATE OR REPLACE PACKAGE pkg_getrecord
    IS
    TYPE myrctype IS REF CURSOR;
    END pkg_getrecord;
    /

    3）. 建立动态sql交叉表函数，输出结果集

    CREATE OR REPLACE FUNCTION fn_rs
    RETURN pkg_getrecord.myrctype
    IS
    s VARCHAR2 (4000);
    CURSOR c1 IS
    SELECT ',sum(case when Class1='
    || class1
    || ' then CallCount else 0 end)'
    || ' "CallCount'
    || class1
    || '"' c2
    FROM t
    GROUP BY class1;
    r1 c1%ROWTYPE;
    list_cursor pkg_getrecord.myrctype;
    BEGIN
    s := 'select CallDate ';
    OPEN c1;
    LOOP
    FETCH c1 INTO r1;
    EXIT WHEN c1%NOTFOUND;
    s := s || r1.c2;
    END LOOP;
    CLOSE c1;
    s := s || ' from T group by CallDate order by CallDate desc ';
    OPEN list_cursor FOR s;
    RETURN list_cursor;
    END fn_rs;
    /

    4）. 测试在sql plus下执行：

    var results refcursor;
    exec :results := fn_rs;
    print results;
    CALLDATE CallCount1 CallCount2 CallCount3
    --------------- ---------- ---------- ----------
    2005-08-09 0 0 33
    2005-08-08 40 77 9
    2005-08-07 6 0 21

    4、不分组只简单行列互换，并且每组之间用虚线分割，可以使用procedure，代码如下:

    create or replace
    procedure print_table(p_query in varchar2)
    authid current_user as
    l_thecursor integer default dbms_sql.open_cursor;
    l_columnvalue varchar2(4000);
    l_status integer;
    l_desctbl dbms_sql.desc_tab;
    l_colcnt number;
    begin
    dbms_sql.parse(l_thecursor,p_query,dbms_sql.native);
    dbms_sql.describe_columns(l_thecursor,l_colcnt,l_desctbl);
    for i in 1..l_colcnt loop
    dbms_sql.define_column(l_thecursor,i,l_columnvalue,4000);
    end loop;
    l_status := dbms_sql.execute(l_thecursor);
    while(dbms_sql.fetch_rows(l_thecursor)>0) loop
    for i in 1..l_colcnt loop
    dbms_sql.column_value(l_thecursor,i,l_columnvalue);
    dbms_output.put_line(rpad(l_desctbl(i).col_name,30)||l_c
    olumnvalue);
    end loop;
    dbms_output.put_line('----------------------------------------------
    -------');
    end loop;
    exception
    when others then
    dbms_sql.close_cursor(l_thecursor);
    raise;
    null;
    end;
    /