Rorohiko, Pūmanawa
SQL Coalesce: whakaahuatanga, tauira whakamahi
A, no te faaineine SQL-pātai maha whakatika te āhuatanga i te mea e tika ana ki te whakarite i te uara o tīwae maha, me te putanga i ratou ina te tauira raraunga, e ngā raraunga (e kore e kau). solves te reira i tenei raruraru maitai faaiteraa Coalesce. I roto i tenei tuhinga ka kitea e koe i te whakaahuatanga ki tonu o te faaiteraa Coalesce SQL, whakaahuatanga o te whakamahinga, me tauira.
Huaputa o uara kore-kau ki Coalesce. āhuatanga
A feruri i Sql Coalesce whakamahi ngā:
- taea te reira ki te whakapūtā tetahi maha o ngā tohenga (pērā Isnull / Nvl / Nvl2, i roto i nei iti te maha o ngā tohenga te).
- Ka taea e te tango te reira i rite te subqueries tautohe.
- hua rite ki te uara tuatahi atu atu Kore Kore ranei hoki, ka taea e te mea e te tahi atu atu i te uara Null kia kitea.
- Ka taea te whakamahi SQL Coalesce ki te tīpako i te uara kupu putuakore Tīpakohia, me te ki te whakamahine hea e kore te e kau uara tīwae huinga tukua (/ whakaaetia).
- Ko ōrite ki te whakamahi o te Case kīanga tenei faaiteraa, tirohia raupapa ia tautohe i runga i te huru, no te kore he null argument1 ka argument1. I roto i te meka, Coalesce ko he "pokatata", hangaia hoki noho pai o te whakamahi, me te i roto i te maha pātengi rapu optimizers tuhituhi anō Coalesce Case kīanga.
- E wātea ana i roto i te nui tūhonohono katoa SQL Coalesce mahi pūnaha whakahaere pātengi raraunga.
wetereo Coalesce
Tangata e kua whakamahia ake te Coalesce i roto i te faaineineraa SQL-pātai, mohio e te wetereo o te faaiteraa he tino māmā. Kati te reira i roto i ngā reu tohu e takina nga tautohetohe mo Kore, wehea e ngā piko. Ki te amo tatou e te tautohetohe ko te arg1 ingoa, arg2, ... argN, Coalesce ka waiho i te wetereo e whai ake:
Coalesce (arg1, arg2, ... argN).
Whakapaia te torutoru tepu mo ako i te huarahi o te mahi o tenei faaiteraa.
tepu faaineineraa
Hei mahino pai Coalesce whakaahuatanga SQL, waihanga i te pātengi raraunga e rua nga papa o te raraunga e whakarato mōhiohio i runga i hoko whare.
Kei roto i te tepu tuatahi nga ingoa Area kia āhuatanga me ratou rohe. kia kia parakore te rohe (area_yt) whakapuakina (area_decl) ranei.
id | object_name | area_yt | area_decl |
1 | whare 1 | 116,2 | 114 |
2 | hanga kia oti 1 | 568 | |
3 | ruma 1 | 64,7 | |
4 | Ko te ruma 2 | 34.2 | |
5 | Tā 1 | 112 | 111,6 |
6 | hanga o 1 | ||
7 | Rūma 3 | 27.9 | |
8 | hanga o 2 | 37,2 | 36.4 |
9 | whare 2 |
tukua Basic_characteristic te tepu tuarua Kei ngā mōhiohio i runga i te āhuatanga matua o te taonga - toronga (Extension), hōhonu (Hōhonu), rohe (Area), rōrahi (Scope), teitei (Teitei).
id | object_name | toronga | hōhonu | Horahanga | Scope | teitei |
1 | whare 1 | 892,4 | 30 | |||
2 | whare 2 | 48 | ||||
3 | hanga o 1 | 164,7 | ||||
4 | Tā 1 | |||||
5 | ruma 1 | 23.6 | ||||
6 | Ko te ruma 2 | 34.7 | ||||
7 | Rūma 3 | 19.8 |
Arotakengia Coalesce whakaahuatanga wetereo SQL, te whakamahinga, me te puta tika ki nga tauira.
tauira o te whakamahi
Wetereo i faaiteraa Coalesce he tino māmā, engari he mea nui e kore e ki te wareware e te hua o te whakahau, ka hei te uara kore-kau tuatahi kitea i roto i te rārangi tohenga. Ko te tino nui tenei kitea, na te tautohetohe i roto i te faaiteraa me whakatakotoria i roto i te raupapa o te faufaa. Ko te huarahi māmā ki te matau i te parau tumu o te wāhi te tepu. Waihanga i te uiui e kōwhiria e te ingoa o te taonga, me te pai kia rite ki te uara o te rohe:
WHIRIWHIRI I Area.id, Area.object_name, coalesce (Area.area_yt, Area.area_decl) MEI Horahanga |
Na te tiki i te hua:
id | object_name | Coalesce |
1 | whare 1 | 116,2 |
2 | hanga kia oti 1 | 568 |
3 | ruma 1 | 64,7 |
4 | Ko te ruma 2 | 34.2 |
5 | Tā 1 | 112 |
6 | hanga o 1 | |
7 | Rūma 3 | 27.9 |
8 | hanga o 2 | 37,2 |
9 | whare 2 |
Hoki te kaupapa "Building 1", "Tā 1" me te "Hanganga 2" Ko i ki tonu te tapawha, engari parakore te rohe i roto i te matua, ka rite ki tohu matou i te reira i roto i te rārangi o te tautohe tuatahi uara e rua. kitea Expression Coalesce te uara tuatahi kore-whakakahore, a kawea ana ia ki te mutu atu tohenga mātakitaki. Tenei hanga o te uiui - te tika, no te mea wāhi motuhake parakore nui atu i te whakapuakina. Ki te mea kua tohua matou rite te tautohe tuatahi ki te rohe whakaaturia, ina whakakī i tenei mara o e kua reira kua he kaupapa matua te tepu.
I tua atu ki te whakamahi i roto i te Tīpakohia, maha Coalesce whakamahia faaiteraa te ki te hea huru. āhei te reira ki te hatepea atu i te hua o te rarangi i roto i nei te uara o te rārangi o ngā āpure pātea (ki te hurihia ranei, ngā i roto i te hua anake aua uara kei hea he kau te rārangi mara). kitea ana tēnei āhuatanga te mau vahi atoa: hei tauira, i roto i te hinonga ki te hoahoa o te kaimahi hou ki te pātengi raraunga i hanga anake ngā mōhiohio taketake e pā ana ki taua mea, me te whakakī i te kōrero i mahue "mo muri". Ina e, te "āputa" puta - ranei i mua o te haki, i te aronga o te kaimahi i runga i tau faafaaearaa / mahi haerenga / hōhipera ranei.
Tīpakohia i te tepu i nga āhuatanga matua o ngā āhuatanga e whai tetahi o nga uara āhuatanga ki te kia whakahoki:
id Tīpakohia, object_name MEI Basic_characteristic Hea coalesce (Toronga, Hōhonu, Area, Scope, Teitei) he korenga |
E tenei tono i roto i te tepu i reira he kotahi kēmu - te ahanoa "Tā 1", nga mara katoa i roto i ai nga āhuatanga o te wātea:
id | object_name |
4 | Tā 1 |
Te ti'aturi nei tatou e to tatou whakamārama āmiki Coalesce SQL te tauturu matau koutou nga āhuatanga katoa o te whakamahi o tenei whakapuaki, me te mahi ki te onäianei nui.
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