，EXPLAIN分析
没有建立索引之前。分析都是表扫描：
sqlite> EXPLAIN SELECT * FROM COMPANY WHERE Salary < 20000;
addr opcode p1 p2 p3 p4 p5 comment
---------- ---------- ---------- ---------- ---------- ---------- ---------- ----------
0 Trace 0 0 0 00
1 Integer 20000 1 0 00
2 Goto 0 16 0 00
3 OpenRead 0 2 0 5 00
4 Rewind 0 14 0 00
5 Column 0 4 2 00
6 Ge 1 13 2 collseq(BI 6b
7 Column 0 0 4 00
8 Column 0 1 5 00
9 Column 0 2 6 00
10 Column 0 3 7 00
11 Column 0 4 8 00
12 ResultRow 4 5 0 00
13 Next 0 5 0 01
14 Close 0 0 0 00
15 Halt 0 0 0 00
16 Transactio 0 0 0 00
17 VerifyCook 0 1 0 00
18 TableLock 0 2 0 COMPANY 00
19 Goto 0 3 0 00
sqlite> EXPLAIN QUERY PLAN SELECT * FROM COMPANY WHERE Salary < 20000;
order from detail
---------- ---------- -------------
0 0 TABLE COMPANY
sqlite>
建立索引。再进行EXPLAIN分析查看结果，走了idx_sal索引扫描：
sqlite> CREATE INDEX idx_sal ON COMPANY(SALARY);
sqlite> EXPLAIN SELECT * FROM COMPANY WHERE Salary < 20000;
addr opcode p1 p2 p3 p4 p5 comment
---------- ---------- ---------- ---------- ---------- ---------- ---------- ----------
0 Trace 0 0 0 00
1 Integer 20000 1 0 00
2 Goto 0 25 0 00
3 OpenRead 0 2 0 5 00
4 OpenRead 1 3 0 keyinfo(1, 00
5 Affinity 2 0 0 cb 00
6 Rewind 1 22 2 0 00
7 SCopy 1 2 0 00
8 IsNull 2 22 0 00
9 Affinity 2 1 0 cb 00
10 IdxGE 1 22 2 1 00
11 Column 1 0 3 00
12 IsNull 3 21 0 00
13 IdxRowid 1 3 0 00
14 Seek 0 3 0 00
15 Column 0 0 4 00
16 Column 0 1 5 00
17 Column 0 2 6 00
18 Column 0 3 7 00
19 Column 1 0 8 00
20 ResultRow 4 5 0 00
21 Next 1 10 0 00
22 Close 0 0 0 00
23 Close 1 0 0 00
24 Halt 0 0 0 00
25 Transactio 0 0 0 00
26 VerifyCook 0 2 0 00
27 TableLock 0 2 0 COMPANY 00
28 Goto 0 3 0 00
sqlite> EXPLAIN QUERY PLAN SELECT * FROM COMPANY WHERE Salary < 20000;
order from detail
---------- ---------- --------------------------------
0 0 TABLE COMPANY WITH INDEX idx_sal
sqlite>
9。删除反复数据而且保留最新一条记录
录入測试数据
sqlite> .dump
PRAGMA foreign_keys=OFF;
BEGIN TRANSACTION;
CREATE TABLE COMPANY(ID INT NOT NULL, NAME VARCHAR(20),AGE INT,ADDRESS VARCHAR(20),SALARY DECIMAL(7,2));
INSERT INTO "COMPANY" VALUES(2,'Allen',25,'Texas',15000);
INSERT INTO "COMPANY" VALUES(3,'Teddy',23,'Norway',20000);
INSERT INTO "COMPANY" VALUES(4,'Mark',25,'Rich-Mond',65000);
INSERT INTO "COMPANY" VALUES(5,'David',27,'Texas',85000);
INSERT INTO "COMPANY" VALUES(6,'Kim',22,'South-Hall',45000);
INSERT INTO "COMPANY" VALUES(7,'James',24,'Houston',10000);
INSERT INTO "COMPANY" VALUES(7,'James',28,'Houston',20000);
INSERT INTO "COMPANY" VALUES(4,'Mark',29,'Rich-Mond',95000);
COMMIT;
sqlite>
查看反复记录数
sqlite> select * from company order by name;
ID NAME AGE ADDRESS SALARY
---------- ---------- ---------- ---------- ----------
2 Allen 25 Texas 15000
5 David 27 Texas 85000
7 James 24 Houston 10000
7 James 28 Houston 20000
6 Kim 22 South-Hall 45000
4 Mark 25 Rich-Mond 65000
4 Mark 29 Rich-Mond 95000
3 Teddy 23 Norway 20000
sqlite>
通过rowid来删除反复记录
sqlite> DELETE FROM COMPANY WHERE rowid NOT IN(SELECT MAX(rowid) rowid FROM COMPANY GROUP BY NAME);
sqlite>
再查看最新的数据记录，已经删除了反复NAME的记录
sqlite> select * from company;
ID NAME AGE ADDRESS SALARY
---------- ---------- ---------- ---------- ----------
2 Allen 25 Texas 15000
3 Teddy 23 Norway 20000
5 David 27 Texas 85000
6 Kim 22 South-Hall 45000
7 James 28 Houston 20000
4 Mark 29 Rich-Mond 95000
sqlite>
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<版权全部。文章同意转载，但必须以链接方式注明源地址，否则追究法律责任!>
原博客地址：http://blog.csdn.net/mchdba/article/details/39826365
原作者：黄杉 (mchdba)
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參考文章：http://www.w3cschool.cc/sqlite/sqlite-tutorial.html
版权声明：本文博客原创文章，博客，未经同意，不得转载。