Kunzwisisa Kushandiswa kweNULLs Kudzivirira Matambudziko Epa Database
Vashandi vatsva kunyika yenhoroondo dzinowanzovhiringidzwa nehukuru hunokosha pamunda - kukosha kweNULL. Ichi chinokosha chinogona kuwanika mumunda waiva nemhando ipi neipi ye deta uye ine chinonyanya kukosha mukati memamiriro ezvinhu ehupfumi hwemashoko. Zvichida zvakanakisisa kutanga kukurukura kwedu kweNULL nemashoko mashomanana pamusoro pekuti NULL haisi iyo :
- NULL haisi nhamba yero.
- NULL haisi tsamba isina chinhu ("") inokosha.
Pane kudaro, NULL inokosha inoshandiswa kureva chimwe chinhu chisingazivikanwi che data. Kazhinji, vateereri vepa database vanoshandisa mutsara we "NULL kukosha", asi izvi hazvina kururama. Yeuka: NULL inokosha isingazivikanwi umo munda unooneka usina chinhu.
NULL muNyika Yechokwadi
Ngationei muenzaniso wakajeka: tafura ine zvinyorwa zvechigadziko chemichero. Ngatitii zvinyorwa zvedu zvine maapuro 10 uye maaroni matatu. Isu tinowedzerawo plums, asi zvinyorwa zvedu zvekuverenga hazvina kukwana uye hatizivi kuti vangani (kana pane) plums iri muzvitoro. Tichishandisa kukosha kweNULL, tinenge tine tafura yekuverenga inoratidzwa patafura iri pasi apa.
Zvibereko Mira Inventory
| InventoryID | Item | Kuwanda |
| 1 | Apples | 10 |
| 2 | Oranges | 3 |
| 3 | Plums | NULL |
Zvinenge zvakajeka kuti hazvina kukodzera kuisa huwandu hwe 0 kune rekodi yeplums, nokuti izvozvo zvinoreva kuti taisava nepums mukutora. Pane zvakasiyana, tinogona kunge tine plums, asi hatisi chokwadi.
KuNULL kana HERE NULL?
Tafura inogona kugadzirirwa kuti inobvumira NULL maitiro kana kwete.
Heino mufananidzo weSQL unogadzira tafura yeInventory iyo inobvumira dzimwe NULLs:
SQL> BASA TABLE INVENTORY (InventoryID INT NOT NULL, Item VARCHAR (20) HERE NULL, Nhamba IN INT;Tafura yeInventory pano haibvumiri NULL maitiro eInventoryID nemikoromo yeItem , asi inovabvumira kuWorld column.
Kunyange zvazvo uchitendera kukosha kweNULL kwakanyatsonaka, NULL maitiro anogona kukonzera matambudziko nekuti chero kuenzanisa kwezvinhu zvayo iyo NULL nguva dzose kunogumira kuNULL.
Kuti uone kana tafura yako ine NULL maitiro, shandisa IS NULL kana HAI NULL operator. Heino muenzaniso weI NULL:
SQL> SHEBA INVENTORYID, ITEM, QUANTITY FROM INVENTORY WHERE QUANTITY HAI NULL;Tipe muenzaniso wedu pano, izvi zvaizodzoka:
| InventoryID | Item | Kuwanda |
|---|---|---|
| 3 | Plums |
Inoshanda paNULLs
Kushanda nemaitiro eNULL kunowanzobereka zviwanikwa zveNULL, zvichienderana nehutano hweSQL . Somuenzaniso, kufunga kuti A ni NULL:
Arithmetic Operators
- A + B = NULL
- A - B = NULL
- A * B = NULL
- A / B = NULL
Kuenzanisa Operators
- A = B = NULL
- A = = B = NULL
- A> B = NULL
- A!
Idzi ingongova mimwe mienzaniso yevashandi iyo ichadzoka nguva dzose NULL kana mumwe opanda ari NULL. Zvimwe zvakawanda zvakaoma mibvunzo zviripo, uye zvose zvakaoma nehutano hweNULL. Iko kunotora kumusha kunoreva kuti, kana iwe uchibvumira NULL maitiro mu database yako, unzwisise zvinorehwa uye zvirongwa kwavari.
Ndiyo NULL muchidimbu!