Oracle查詢優化--單表查詢

--查詢所有

select * from emp;
select * from emp where comm is null;

--錯誤表達

--select * from emp where comm = null;

--其他有關null的返回結果

select replace('abcde','a',null) as str from dual;
select greatest(1,null) from dual;

/*結論：遇到null最好先測試一下，不能臆想猜測*/

--coalesce函數

SELECT coalesce(comm,0) FROM emp;

--上述例子中coalesce與nvl函數起同樣作用，但coalesce函數更好用，支持更多參數，能很方便地返回第一個不為空的值

CREATE OR REPLACE VIEW v AS 
SELECT NULL AS c1,NULL AS c2,'1' AS c3,NULL AS c4,'2' AS c5,NULL AS c6 FROM dual UNION ALL
SELECT NULL AS c1,NULL AS c2,NULL AS c3,'3' AS c4,NULL AS c5,'2' AS c6 FROM dual;
SELECT * FROM v; 
SELECT COALESCE(c1,c2,c3,c4,c5,c6) AS c FROM v;

--key:別名

--desc:可以為結果集的列指定別名，用AS或空格緊跟。
SELECT 姓名 FROM (SELECT ename 姓名,comm AS 提成 FROM emp) WHERE 提成 IS NULL;
--summary:使用別名當篩選條件需要在外面嵌套一層
--key:拼接
--desc:concat和||都可以拼接字段

SELECT ename || '的工作是' ||job AS msg FROM emp WHERE deptno=20;
SELECT concat('姓名：',ename) AS msg FROM emp WHERE deptno=20;

--summary:concat可以拼接兩個字段，使用||可以拼接多重字段

--key:生成sql
--desc:用sql生成sql

DROP TABLE test_concat PURGE;
CREATE TABLE test_concat AS 
SELECT table_name,
'N_' || table_name AS new_tbl_name,
column_name,
'new_' || column_name AS new_col_name
FROM all_tab_cols
WHERE owner = 'SCOTT';

/*博客鏈接*/

DECLARE v_sql CLOB;
BEGIN
FOR cur IN (SELECT 'CREATE OR REPLACE VIEW ' || new_tbl_name || ' as ' || chr(10) ||
'select ' || chr(10) ||
wmsys.wm_concat(column_name || ' as ' || new_col_name || chr(10)) ||
'from scott.' || table_name AS create_view
FROM test_concat
GROUP BY table_name,new_tbl_name)
LOOP
v_sql := cur.create_view;
EXECUTE IMMEDIATE v_sql;
END LOOP;
END;

 

--key:條件邏輯

--desc:case when

SELECT (CASE 
WHEN sal<1000 THEN '0000-1000' 
WHEN sal<=2000 THEN '1000-2000'
ELSE '好高' END) AS 提成 
FROM emp;

--key:rownum

--desc:限制返回行數

SELECT * FROM emp WHERE ROWNUM<=3 ORDER BY sal DESC;

--upgrade

SELECT * FROM (SELECT ROWNUM rn ,emp.* FROM emp WHERE ROWNUM<=3) WHERE rn=2;

--summary:並不能通過order by和rownum的配合得到排名

--key:隨機
--desc:隨機抽查3行數據

SELECT empno,ename FROM (SELECT dbms_random.value(),empno,ename FROM emp ORDER BY dbms_random.value()) WHERE ROWNUM <= 3;

/*summary:必須嵌套使用，否則直接使用並不能得到隨機效果：

SELECT dbms_random.value(),empno,ename FROM emp where rownum <=3 ORDER BY dbms_random.value()

因為查詢語句中執行的順序是：！.select 2.rownum 3.order by 

所以，在order by執行之前，前三行已經定了，隨后對已選定的三行進行隨機顯示而已*/
--key:模糊查詢
--desc:模糊查詢及轉義字符的使用
--prep:

CREATE OR REPLACE VIEW v AS SELECT 'ABCEDF' AS vname FROM dual
UNION ALL
SELECT '_BCEFG' AS vname FROM dual
UNION ALL
SELECT '_BCEDF' AS vname FROM dual
UNION ALL
SELECT '_\BCEDF' AS vname FROM dual
UNION ALL
SELECT 'XYCEG' AS vname FROM dual
SELECT * FROM v;

--查詢包含CED的行

SELECT * FROM v WHERE vname LIKE '%CED%';

--查詢包含_BCE的行

SELECT * FROM v WHERE vname LIKE '%_BCE%';--ABCEDF和_\BCEDF並不是想要的結果，因為_被當做通配符了
SELECT * FROM v WHERE vname LIKE '%\_BCE%' ESCAPE '\';--escape是用來定義轉義字符的，只有定義了才有效果，也可以定義成其他，如*、/等