一、decode语法

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SELECT deptno,       nvl(SUM(decode(job, 'MANAGER', sal)), 0) s_MANAGER,       nvl(SUM(decode(job, 'ANALYST', sal)), 0) s_ANALYST,       nvl(SUM(decode(job, 'CLERK', sal)), 0) s_CLERK,       nvl(SUM(decode(job, 'PRESIDENT', sal)), 0) s_PRESIDENT,       nvl(SUM(decode(job, 'SALESMAN', sal)), 0) s_SALESMAN  FROM empGROUP BY deptno;

二、CASE语法

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SELECT deptno,       nvl(sum(case when job = 'MANAGER' then sal else 0 end),0)  s_MANAGER,       nvl(sum(case when job = 'ANALYST' then sal else 0 end),0)  s_ANALYST,       nvl(sum(case when job = 'CLERK' then sal else 0 end),0)    s_CLERK,       nvl(sum(case when job = 'PRESIDENT' then sal else 0 end),0)  s_PRESIDENT,       nvl(sum(case when job = 'SALESMAN' then sal else 0 end),0)  s_SALESMAN  FROM emp GROUP BY deptno;

 

三、PIVOT语法

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WITH p AS (SELECT deptno, job, sal FROM emp)SELECT *  FROM p pivot(SUM(sal) FOR job IN('MANAGER' AS s_MANAGER,                                   'ANALYST' AS s_ANALYST,                                   'CLERK' AS s_CLERK,                                   'PRESIDENT' AS s_PRESIDENT,                                   'SALESMAN' AS s_SALESMAN));

 

不过这个地方null值没有替换成0，要通过nvl再转换一下。

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WITH p AS (SELECT deptno, job, sal FROM emp),tmp AS (SELECT *    FROM p pivot(SUM(sal) FOR job IN('MANAGER' AS s_MANAGER,                                     'ANALYST' AS s_ANALYST,                                     'CLERK' AS s_CLERK,                                     'PRESIDENT' AS s_PRESIDENT,                                     'SALESMAN' AS s_SALESMAN)))SELECT deptno,       nvl(s_MANAGER, 0) s_MANAGER,       nvl(s_ANALYST, 0) s_ANALYST,       nvl(s_CLERK, 0) s_CLERK,       nvl(s_PRESIDENT, 0) s_PRESIDENT,       nvl(s_SALESMAN, 0) s_SALESMAN  FROM tmp

 

小结：

decode 语法简单，Oracle独有。
case sql标准语法。
pivot 语法最为简单，Oracle、sqlserver、postgresql均可以使用。

 

下面再来讲讲wm_concat、listagg、xmlagg。

 

需求：部门编号为20的所有的员工信息，以行的形式显示。

 

四、wm_contact语法

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SELECT T.DEPTNO, wm_concat(t.ename) names  FROM EMP T WHERE T.DEPTNO = '20' GROUP BY T.DEPTNO;

 

 

五、listagg语法

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SELECT T.DEPTNO,       listagg(T.ENAME, ',') WITHIN GROUP(ORDER BY T .ENAME) names  FROM EMP T WHERE T.DEPTNO = '20' GROUP BY T.DEPTNO;

六、xmlagg语法

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SELECT T.DEPTNO,       xmlagg(XMLELEMENT(T, ',',T.ENAME) ORDER BY T .ENAME).EXTRACT ('//text()') names  FROM EMP T WHERE T.DEPTNO = '20' GROUP BY T.DEPTNO;

 

小结：

wm_concat 语法最简单，但是默认是 clob列，plsql不容易导出。
listtagg 语法稍微复杂，但是默认是字符串，性能会比wm_concat 好，但是超过4000个字符，受限制。
xmlagg 字符串超过4000字符,就需要使用xmlagg