Oracle SQL精妙SQL语句讲解

Posted 2020-09-20

- 如果存在就更新，不存在就插入用一个语句实现 
DROP TABLE t_mg; 
CREATE TABLE t_mg(code VARCHAR2(10), NAME VARCHAR2(10)); 

SELECT * FROM t_mg; 

MERGE INTO t_mg a 
USING (SELECT ‘the code‘ code, ‘the name‘ NAME FROM dual) b 
ON (a.code = b.code) 
WHEN MATCHED THEN 
UPDATE SET a.NAME = b.NAME 
WHEN NOT MATCHED THEN 
INSERT (code, NAME) VALUES (b.code, b.NAME); 

-- 抽取/删除重复记录 
DROP TABLE t_dup; 
CREATE TABLE t_dup AS SELECT ‘code_‘||ROWNUM code, dbms_random.string(‘z‘,5)NAME FROM dual CONNECT BY ROWNUM<=10; 
INSERT INTO t_dup SELECT ‘code_‘||ROWNUM code, dbms_random.string(‘z‘,5) NAMEFROM dual CONNECT BY ROWNUM<=2; 

SELECT * FROM t_dup; 

SELECT * FROM t_dup aWHERE a.ROWID <> (SELECT MIN(b.ROWID) FROM t_dup b WHERE a.code=b.code); 

SELECT b.code, b.NAME 
FROM (SELECT a.code, 
a.NAME, 
row_number() over(PARTITION BY a.code ORDER BY a.ROWID) rn 
FROM t_dup a) b 
WHERE b.rn > 1; 

-- IN/EXISTS的不同适用环境 
-- t_orders.customer_id有索引 
SELECT a.* 
FROM t_employees a 
WHERE a.employee_id IN 
(SELECT b.sales_rep_id FROM t_orders b WHERE b.customer_id = 12); 

SELECT a.* 
FROM t_employees a 
WHERE EXISTS (SELECT 1 
FROM t_orders b 
WHERE b.customer_id = 12 
AND a.employee_id = b.sales_rep_id); 

--t_employees.department_id有索引 
SELECT a.* 
FROM t_employees a 
WHERE a.department_id = 10 
AND EXISTS 
(SELECT 1 FROM t_orders b WHERE a.employee_id = b.sales_rep_id); 

SELECT a.* 
FROM t_employees a 
WHERE a.department_id = 10 
AND a.employee_id IN (SELECT b.sales_rep_id FROM t_orders b); 

-- FBI 
DROP TABLE t_fbi; 
CREATE TABLE t_fbi AS 
SELECT ROWNUM rn, dbms_random.STRING(‘z‘,10) NAME , SYSDATE + dbms_random.VALUE* 10 dt FROM dual 
CONNECT BY ROWNUM <=10; 

CREATE INDEX idx_nonfbiON t_fbi(dt); 

DROP INDEX idx_fbi_1; 
CREATE INDEX idx_fbi_1 ON t_fbi(trunc(dt)); 

SELECT * FROM t_fbiWHERE trunc(dt) = to_date(‘2006-09-21‘,‘yyyy-mm-dd‘) ; 

-- 不建议使用 
SELECT * FROM t_fbi WHERE to_char(dt, ‘yyyy-mm-dd‘) = ‘2006-09-21‘; 

-- LOOP中的COMMIT/ROLLBACK 
DROP TABLE t_loop PURGE; 
create TABLE t_loop AS SELECT * FROM user_objects WHERE 1=2; 

SELECT * FROM t_loop; 

-- 逐行提交 
DECLARE 
BEGIN 
FOR cur IN (SELECT * FROM user_objects) LOOP 
INSERT INTO t_loop VALUES cur; 
COMMIT; 
END LOOP; 
END;

-- 模拟批量提交
DECLARE 
v_count NUMBER; 
BEGIN 
FOR cur IN (SELECT * FROM user_objects) LOOP 
INSERT INTO t_loop VALUES cur; 
v_count := v_count + 1; 
IF v_count >= 100 THEN 
COMMIT; 
END IF; 
END LOOP; 
COMMIT; 
END; 

-- 真正的批量提交 
DECLARE 
CURSOR cur IS 
SELECT * FROM user_objects; 
TYPE rec IS TABLE OF user_objects%ROWTYPE; 
recs rec; 
BEGIN 
OPEN cur; 
WHILE (TRUE) LOOP 
FETCH cur BULK COLLECT 
INTO recs LIMIT 100; 
-- forall 实现批量 
FORALL i IN 1 .. recs.COUNT 
INSERT INTO t_loop VALUES recs (i); 
COMMIT; 
EXIT WHEN cur%NOTFOUND; 
END LOOP; 
CLOSE cur; 
END;