Union kev txiav txim los ntawm cov lus nug syntax / nqe lus siv hauv MySQL database

MySQL database UNION ORDER BY kev siv cov lus nug syntax/lus qhia

MySQL UNION tus neeg ua haujlwm

Qhov kev qhia no qhia koj txog cov syntax thiab cov piv txwv ntawm MySQL UNION tus neeg teb xov tooj.

描述

Tus neeg teb xov tooj MySQL UNION yog siv los ua ke cov txiaj ntsig ntawm ob lossis ntau dua SELECT cov lus rau hauv ib qho txiaj ntsig.Ntau cov lus SELECT tshem tawm cov ntaub ntawv sib npaug.

qauv sau ntawv

MySQL UNION tus neeg teb xov tooj syntax hom:

SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions]
UNION [ALL | DISTINCT]
SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions];

Tsis

• expression1, expression2, ... expression_n: Kem retrieve.
• rooj: Cov ntaub ntawv cov ntaub ntawv mus retrieve.
• QHOV CHAW UA HAUJ LWM: Yeem xaiv, tshawb xyuas cov txheej txheem.
• DISTINCT: Optionally, tshem tawm duplicate cov ntaub ntawv los ntawm qhov tshwm sim.Tus neeg teb xov tooj UNION tau deduplicated cov ntaub ntawv los ntawm lub neej ntawd, yog li tus DISTINCT modifier tsis cuam tshuam rau qhov tshwm sim.
• TXHUA: Yeem, rov qab tag nrho cov txiaj ntsig, suav nrog cov duplicates.

demo database

Nyob rau hauv no nyeem, peb yuav siv chenweiliang qauv database.

Nov yog cov ntaub ntawv los ntawm lub rooj "Websites":

mysql> SELECT * FROM Websites;
+----+--------------+---------------------------+-------+---------+
| id | name         | url                       | alexa | country |
+----+--------------+---------------------------+-------+---------+
| 1  | Google       | https://www.google.cm/    | 1     | USA     |
| 2  | 淘宝          | https://www.taobao.com/   | 13    | CN      |
| 3  | 陈沩亮博客      | http://www.chenweiliang.com/    | 4689  | CN      |
| 4  | 微博          | http://weibo.com/         | 20    | CN      |
| 5  | Facebook     | https://www.facebook.com/ | 3     | USA     |
| 7  | stackoverflow | http://stackoverflow.com/ |   0 | IND     |
+----+---------------+---------------------------+-------+---------+

Nov yog cov ntaub ntawv rau "apps" APP:

mysql> SELECT * FROM apps;
+----+------------+-------------------------+---------+
| id | app_name   | url                     | country |
+----+------------+-------------------------+---------+
|  1 | QQ APP     | http://im.qq.com/       | CN      |
|  2 | 微博 APP | http://weibo.com/       | CN      |
|  3 | 淘宝 APP | https://www.taobao.com/ | CN      |
+----+------------+-------------------------+---------+
3 rows in set (0.00 sec)

SQL UNION piv txwv

Cov lus qhia SQL hauv qab no xaiv txhua lub tebchaws sib txawv (tsuas yog cov nqi sib txawv) los ntawm cov lus "Websites" thiab "apps" :

SELECT country FROM Websites
UNION
SELECT country FROM apps
ORDER BY country;

SQL UNION TXHUA qhov piv txwv

Cov lus qhia SQL hauv qab no siv UNION ALL los xaiv txhua lub tebchaws (suav nrog cov nqi theej) los ntawm cov lus "Websites" thiab "apps" :

SELECT country FROM Websites
UNION ALL
SELECT country FROM apps
ORDER BY country;

SQL UNION TXHUA YAM nrog qhov twg

Cov lus qhia SQL hauv qab no siv UNION ALL los xaiv tag nrho cov ntaub ntawv rau Tuam Tshoj (CN) los ntawm cov lus "Websites" thiab "apps" (suav nrog cov nqi duplicate):

SELECT country, name FROM Websites
WHERE country='CN'
UNION ALL
SELECT country, app_name FROM apps
WHERE country='CN'
ORDER BY country;