Jeg er AI-forsker, så en af de vigtigste ting, jeg beskæftiger mig med, er data. En masse af det.
Med mere end 2,5 exabyte data genereret hver dag , det kommer ikke som nogen overraskelse, at disse data skal gemmes et sted, hvor vi kan få adgang til dem, når vi har brug for dem.
Denne artikel vil lede dig gennem et snydeark, der kan hackes, så du hurtigt kan komme i gang med SQL.
Hvad er SQL?
SQL står for Structured Query Language. Det er et sprog for relationelle databasestyringssystemer. SQL bruges i dag til at gemme, hente og manipulere data i relationelle databaser.
Sådan ser en grundlæggende relationsdatabase ud:
Ved hjælp af SQL kan vi interagere med databasen ved at skrive forespørgsler.
Sådan ser en eksempelforespørgsel ud:
SELECT * FROM customers;
Brug af denne SELECT sætning, vælger forespørgslen alle dataene fra alle kolonnerne i kundens tabel og returnerer data som sådan:
Stjernen jokertegn (*) henviser til "alle ” og vælger alle rækkerne og kolonnerne. Vi kan erstatte det med specifikke kolonnenavne i stedet — her vil kun disse kolonner blive returneret af forespørgslen
SELECT FirstName, LastName FROM customers;
Tilføjelse af en WHERE klausul giver dig mulighed for at filtrere, hvad der returneres:
SELECT * FROM customers WHERE age >= 30 ORDER BY age ASC; Denne forespørgsel returnerer alle data fra produkttabellen med en alder værdi på mere end 30.
Brugen af ORDER BY søgeord betyder blot, at resultaterne vil blive sorteret ved hjælp af alderskolonnen fra den laveste værdi til den højeste
Brug af INSERT INTO sætning, kan vi tilføje nye data til en tabel. Her er et grundlæggende eksempel på tilføjelse af en ny bruger til kundetabellen:
INSERT INTO customers(FirstName, LastName, address, email)
VALUES ('Jason', 'Dsouza', 'McLaren Vale, South Australia', '[email protected]'); Disse eksempler viser naturligvis kun et meget lille udvalg af, hvad SQL-sproget kan. Vi lærer mere om det i denne vejledning.
Hvorfor lære SQL?
Vi lever i Big Datas tidsalder, hvor data bruges flittigt til at finde indsigt og informere om strategi, markedsføring, annoncering og en lang række andre operationer.
Store virksomheder som Google, Amazon, AirBnb bruger store relationelle databaser som grundlag for at forbedre kundeoplevelsen. At forstå SQL er en stor færdighed at have, ikke kun for datavidenskabsmænd og analytikere, men for alle.
Hvordan tror du, at du pludselig fik en Youtube-annonce på sko, da du for bare et par minutter siden googlede dine yndlingssko? Det er SQL (eller en form for SQL) på arbejde!
SQL vs MySQL
Før vi går videre, vil jeg lige præcisere et ofte forvirret emne — forskellen mellem SQL og MySQL. Som det viser sig, er de ikke det samme!
SQL er et sprog, mens MySQL er et system til at implementere SQL.
SQL skitserer syntaks, der giver dig mulighed for at skrive forespørgsler, der administrerer relationsdatabaser.
MySQL er et database system der kører på en server. Det giver dig mulighed for at skrive forespørgsler ved hjælp af SQL-syntaks til at administrere MySQL-databaser.
Udover MySQL er der andre systemer, der implementerer SQL. Nogle af de mere populære omfatter:
- SQLite
- Oracle-database
- PostgreSQL
- Microsoft SQL Server
Sådan installeres MySQL
I de fleste tilfælde er MySQL det foretrukne valg til et databasestyringssystem. Mange populære indholdsstyringssystemer (som Wordpress) bruger MySQL som standard, så det kan være en god idé at bruge MySQL til at administrere disse applikationer.
For at bruge MySQL skal du installere det på dit system:
Installer MySQL på Windows
Den anbefalede måde at installere MySQL på Windows er ved at bruge MSI-installationsprogrammet fra MySQL-webstedet.
Denne ressource vil guide dig med installationsprocessen.
Installer MySQL på macOS
På macOS involverer installation af MySQL også at downloade et installationsprogram.
Denne ressource vil guide dig gennem installationsprocessen.
Sådan bruges MySQL
Med MySQL nu installeret på dit system, anbefaler jeg, at du bruger en form for SQL-administrationsapplikation for at gøre administrationen af dine databaser til en meget lettere proces.
Der er masser af apps at vælge imellem, som stort set udfører det samme arbejde, så det er op til din egen personlige præference, hvilken du skal bruge:
- MySQL Workbench udviklet af Oracle
- phpMyAdmin (fungerer i webbrowseren)
- HeidiSQL (anbefalet til Windows)
- Sequel Pro (anbefales til macOS)
Når du er klar til at begynde at skrive dine egne SQL-forespørgsler, kan du overveje at importere dummy-data i stedet for at oprette din egen database.
Her er nogle dummy-databaser, der kan downloades gratis.
SQL Cheatsheet – prikken over i'et
SQL-søgeord
Her kan du finde en samling nøgleord brugt i SQL-sætninger, en beskrivelse og, hvor det er relevant, et eksempel. Nogle af de mere avancerede søgeord har deres egen dedikerede sektion.
Hvor MySQL er nævnt ved siden af et eksempel, betyder det, at dette eksempel kun gælder for MySQL-databaser (i modsætning til ethvert andet databasesystem).
ADD -- Adds a new column to an existing table
ADD CONSTRAINT -- Creates a new constraint on an existing table, which is used to specify rules for any data in the table.
ALTER TABLE -- Adds, deletes or edits columns in a table. It can also be used to add and delete constraints in a table, as per the above.
ALTER COLUMN -- Changes the data type of a table’s column.
ALL -- Returns true if all of the subquery values meet the passed condition.
AND -- Used to join separate conditions within a WHERE clause.
ANY -- Returns true if any of the subquery values meet the given condition.
AS -- Renames a table or column with an alias value which only exists for the duration of the query.
ASC -- Used with ORDER BY to return the data in ascending order.
BETWEEN -- Selects values within the given range.
CASE -- Changes query output depending on conditions.
CHECK -- Adds a constraint that limits the value which can be added to a column.
CREATE DATABASE -- Creates a new database.
CREATE TABLE -- Creates a new table.
DEFAULT -- Sets a default value for a column
DELETE -- Delete data from a table.
DESC -- Used with ORDER BY to return the data in descending order.
DROP COLUMN -- Deletes a column from a table.
DROP DATABASE -- Deletes the entire database.
DROP DEAFULT -- Removes a default value for a column.
DROP TABLE -- Deletes a table from a database.
EXISTS -- Checks for the existence of any record within the subquery, returning true if one or more records are returned.
FROM -- Specifies which table to select or delete data from.
IN -- Used alongside a WHERE clause as a shorthand for multiple OR conditions.
INSERT INTO -- Adds new rows to a table.
IS NULL -- Tests for empty (NULL) values.
IS NOT NULL -- The reverse of NULL. Tests for values that aren’t empty / NULL.
LIKE -- Returns true if the operand value matches a pattern.
NOT -- Returns true if a record DOESN’T meet the condition.
OR -- Used alongside WHERE to include data when either condition is true.
ORDER BY -- Used to sort the result data in ascending (default) or descending order through the use of ASC or DESC keywords.
ROWNUM -- Returns results where the row number meets the passed condition.
SELECT -- Used to select data from a database, which is then returned in a results set.
SELECT DISTINCT -- Sames as SELECT, except duplicate values are excluded.
SELECT INTO -- Copies data from one table and inserts it into another.
SELECT TOP -- Allows you to return a set number of records to return from a table.
SET -- Used alongside UPDATE to update existing data in a table.
SOME -- Identical to ANY.
TOP -- Used alongside SELECT to return a set number of records from a table.
TRUNCATE TABLE -- Similar to DROP, but instead of deleting the table and its data, this deletes only the data.
UNION -- Combines the results from 2 or more SELECT statements and returns only distinct values.
UNION ALL -- The same as UNION, but includes duplicate values.
UNIQUE -- This constraint ensures all values in a column are unique.
UPDATE -- Updates existing data in a table.
VALUES -- Used alongside the INSERT INTO keyword to add new values to a table.
WHERE -- Filters results to only include data which meets the given condition.
Kommentarer i SQL
Kommentarer giver dig mulighed for at forklare sektioner af dine SQL-sætninger uden at blive udført direkte.
I SQL er der 2 typer kommentarer, enkelt linje og multilinje.
Enkeltlinjekommentarer i SQL
Enkeltlinjekommentarer starter med '- -'. Enhver tekst efter disse 2 tegn til slutningen af linjen vil blive ignoreret.
-- This part is ignored
SELECT * FROM customers; Flere linjekommentarer i SQL
Flerlinjekommentarer starter med /* og slutter med */. De strækker sig over flere linjer, indtil de afsluttende tegn er fundet.
/*
This is a multiline comment.
It can span across multiple lines.
*/
SELECT * FROM customers;
/*
This is another comment.
You can even put code within a comment to prevent its execution
SELECT * FROM icecreams;
*/ Datatyper i MySQL
Når du opretter en ny tabel eller redigerer en eksisterende, skal du angive den type data, som hver kolonne accepterer.
I dette eksempel blev data sendt til id kolonne skal være et int (heltal), mens FirstName kolonne har en VARCHAR datatype med maksimalt 255 tegn.
CREATE TABLE customers(
id int,
FirstName varchar(255)
); 1. Strengdatatyper
CHAR(size) -- Fixed length string which can contain letters, numbers and special characters. The size parameter sets the maximum string length, from 0 – 255 with a default of 1.
VARCHAR(size) -- Variable length string similar to CHAR(), but with a maximum string length range from 0 to 65535.
BINARY(size) -- Similar to CHAR() but stores binary byte strings.
VARBINARY(size) -- Similar to VARCHAR() but for binary byte strings.
TINYBLOB -- Holds Binary Large Objects (BLOBs) with a max length of 255 bytes.
TINYTEXT -- Holds a string with a maximum length of 255 characters. Use VARCHAR() instead, as it’s fetched much faster.
TEXT(size) -- Holds a string with a maximum length of 65535 bytes. Again, better to use VARCHAR().
BLOB(size) -- Holds Binary Large Objects (BLOBs) with a max length of 65535 bytes.
MEDIUMTEXT -- Holds a string with a maximum length of 16,777,215 characters.
MEDIUMBLOB -- Holds Binary Large Objects (BLOBs) with a max length of 16,777,215 bytes.
LONGTEXT -- Holds a string with a maximum length of 4,294,967,295 characters.
LONGBLOB -- Holds Binary Large Objects (BLOBs) with a max length of 4,294,967,295 bytes.
ENUM(a, b, c, etc…) -- A string object that only has one value, which is chosen from a list of values which you define, up to a maximum of 65535 values. If a value is added which isn’t on this list, it’s replaced with a blank value instead.
SET(a, b, c, etc…) -- A string object that can have 0 or more values, which is chosen from a list of values which you define, up to a maximum of 64 values.
2. Numeriske datatyper
BIT(size) -- A bit-value type with a default of 1. The allowed number of bits in a value is set via the size parameter, which can hold values from 1 to 64.
TINYINT(size) -- A very small integer with a signed range of -128 to 127, and an unsigned range of 0 to 255. Here, the size parameter specifies the maximum allowed display width, which is 255.
BOOL -- Essentially a quick way of setting the column to TINYINT with a size of 1. 0 is considered false, whilst 1 is considered true.
BOOLEAN -- Same as BOOL.
SMALLINT(size) -- A small integer with a signed range of -32768 to 32767, and an unsigned range from 0 to 65535. Here, the size parameter specifies the maximum allowed display width, which is 255.
MEDIUMINT(size) -- A medium integer with a signed range of -8388608 to 8388607, and an unsigned range from 0 to 16777215. Here, the size parameter specifies the maximum allowed display width, which is 255.
INT(size) -- A medium integer with a signed range of -2147483648 to 2147483647, and an unsigned range from 0 to 4294967295. Here, the size parameter specifies the maximum allowed display width, which is 255.
INTEGER(size) -- Same as INT.
BIGINT(size) -- A medium integer with a signed range of -9223372036854775808 to 9223372036854775807, and an unsigned range from 0 to 18446744073709551615. Here, the size parameter specifies the maximum allowed display width, which is 255.
FLOAT(p) -- A floating point number value. If the precision (p) parameter is between 0 to 24, then the data type is set to FLOAT(), whilst if it's from 25 to 53, the data type is set to DOUBLE(). This behaviour is to make the storage of values more efficient.
DOUBLE(size, d) -- A floating point number value where the total digits are set by the size parameter, and the number of digits after the decimal point is set by the d parameter.
DECIMAL(size, d) -- An exact fixed point number where the total number of digits is set by the size parameters, and the total number of digits after the decimal point is set by the d parameter.
DEC(size, d) -- Same as DECIMAL. 3. Dato/tid datatyper
DATE -- A simple date in YYYY-MM–DD format, with a supported range from ‘1000-01-01’ to ‘9999-12-31’.
DATETIME(fsp) -- A date time in YYYY-MM-DD hh:mm:ss format, with a supported range from ‘1000-01-01 00:00:00’ to ‘9999-12-31 23:59:59’. By adding DEFAULT and ON UPDATE to the column definition, it automatically sets to the current date/time.
TIMESTAMP(fsp) -- A Unix Timestamp, which is a value relative to the number of seconds since the Unix epoch (‘1970-01-01 00:00:00’ UTC). This has a supported range from ‘1970-01-01 00:00:01’ UTC to ‘2038-01-09 03:14:07’ UTC.
By adding DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT TIMESTAMP to the column definition, it automatically sets to current date/time.
TIME(fsp) -- A time in hh:mm:ss format, with a supported range from ‘-838:59:59’ to ‘838:59:59’.
YEAR -- A year, with a supported range of ‘1901’ to ‘2155’. SQL-operatører
1. Aritmetiske operatorer i SQL
+ -- Add
– -- Subtract
* -- Multiply
/ -- Divide
% -- Modulus 2. Bitwise Operators i SQL
& -- Bitwise AND
| -- Bitwise OR
^-- Bitwise XOR 3. Sammenligningsoperatører i SQL
= -- Equal to
> -- Greater than
< -- Less than
>= -- Greater than or equal to
<= -- Less than or equal to
<> -- Not equal to 4. Sammensatte operatører i SQL
+= -- Add equals
-= -- Subtract equals
*= -- Multiply equals
/= -- Divide equals
%= -- Modulo equals
&= -- Bitwise AND equals
^-= -- Bitwise exclusive equals
|*= -- Bitwise OR equals SQL-funktioner
1. Strengfunktioner i SQL
ASCII -- Returns the equivalent ASCII value for a specific character.
CHAR_LENGTH -- Returns the character length of a string.
CHARACTER_LENGTH -- Same as CHAR_LENGTH.
CONCAT -- Adds expressions together, with a minimum of 2.
CONCAT_WS -- Adds expressions together, but with a separator between each value.
FIELD -- Returns an index value relative to the position of a value within a list of values.
FIND IN SET -- Returns the position of a string in a list of strings.
FORMAT -- When passed a number, returns that number formatted to include commas (eg 3,400,000).
INSERT -- Allows you to insert one string into another at a certain point, for a certain number of characters.
INSTR -- Returns the position of the first time one string appears within another.
LCASE -- Converts a string to lowercase.
LEFT -- Starting from the left, extracts the given number of characters from a string and returns them as another.
LENGTH -- Returns the length of a string, but in bytes.
LOCATE -- Returns the first occurrence of one string within another,
LOWER -- Same as LCASE.
LPAD -- Left pads one string with another, to a specific length.
LTRIM -- Removes any leading spaces from the given string.
MID -- Extracts one string from another, starting from any position.
POSITION -- Returns the position of the first time one substring appears within another.
REPEAT -- Allows you to repeat a string
REPLACE -- Allows you to replace any instances of a substring within a string, with a new substring.
REVERSE -- Reverses the string.
RIGHT -- Starting from the right, extracts the given number of characters from a string and returns them as another.
RPAD -- Right pads one string with another, to a specific length.
RTRIM -- Removes any trailing spaces from the given string.
SPACE -- Returns a string full of spaces equal to the amount you pass it.
STRCMP -- Compares 2 strings for differences
SUBSTR -- Extracts one substring from another, starting from any position.
SUBSTRING -- Same as SUBSTR
SUBSTRING_INDEX -- Returns a substring from a string before the passed substring is found the number of times equals to the passed number.
TRIM -- Removes trailing and leading spaces from the given string. Same as if you were to run LTRIM and RTRIM together.
UCASE -- Converts a string to uppercase.
UPPER -- Same as UCASE. 2. Numeriske funktioner i SQL
ABS -- Returns the absolute value of the given number.
ACOS -- Returns the arc cosine of the given number.
ASIN -- Returns the arc sine of the given number.
ATAN -- Returns the arc tangent of one or 2 given numbers.
ATAN2 -- Returns the arc tangent of 2 given numbers.
AVG -- Returns the average value of the given expression.
CEIL -- Returns the closest whole number (integer) upwards from a given decimal point number.
CEILING -- Same as CEIL.
COS -- Returns the cosine of a given number.
COT -- Returns the cotangent of a given number.
COUNT -- Returns the amount of records that are returned by a SELECT query.
DEGREES -- Converts a radians value to degrees.
DIV -- Allows you to divide integers.
EXP -- Returns e to the power of the given number.
FLOOR -- Returns the closest whole number (integer) downwards from a given decimal point number.
GREATEST -- Returns the highest value in a list of arguments.
LEAST -- Returns the smallest value in a list of arguments.
LN -- Returns the natural logarithm of the given number.
LOG -- Returns the natural logarithm of the given number, or the logarithm of the given number to the given base.
LOG10 -- Does the same as LOG, but to base 10.
LOG2 -- Does the same as LOG, but to base 2.
MAX -- Returns the highest value from a set of values.
MIN -- Returns the lowest value from a set of values.
MOD -- Returns the remainder of the given number divided by the other given number.
PI -- Returns PI.
POW -- Returns the value of the given number raised to the power of the other given number.
POWER -- Same as POW.
RADIANS -- Converts a degrees value to radians.
RAND -- Returns a random number.
ROUND -- Rounds the given number to the given amount of decimal places.
SIGN -- Returns the sign of the given number.
SIN -- Returns the sine of the given number.
SQRT -- Returns the square root of the given number.
SUM -- Returns the value of the given set of values combined.
TAN -- Returns the tangent of the given number.
TRUNCATE -- Returns a number truncated to the given number of decimal places. 3. Datofunktioner i SQL
ADDDATE -- Adds a date interval (eg: 10 DAY) to a date (eg: 20/01/20) and returns the result (eg: 20/01/30).
ADDTIME -- Adds a time interval (eg: 02:00) to a time or datetime (05:00) and returns the result (07:00).
CURDATE -- Gets the current date.
CURRENT_DATE -- Same as CURDATE.
CURRENT_TIME -- Gest the current time.
CURRENT_TIMESTAMP -- Gets the current date and time.
CURTIME -- Same as CURRENT_TIME.
DATE -- Extracts the date from a datetime expression.
DATEDIFF -- Returns the number of days between the 2 given dates.
DATE_ADD -- Same as ADDDATE.
DATE_FORMAT -- Formats the date to the given pattern.
DATE_SUB -- Subtracts a date interval (eg: 10 DAY) to a date (eg: 20/01/20) and returns the result (eg: 20/01/10).
DAY -- Returns the day for the given date.
DAYNAME -- Returns the weekday name for the given date.
DAYOFWEEK -- Returns the index for the weekday for the given date.
DAYOFYEAR -- Returns the day of the year for the given date.
EXTRACT -- Extracts from the date the given part (eg MONTH for 20/01/20 = 01).
FROM DAYS -- Returns the date from the given numeric date value.
HOUR -- Returns the hour from the given date.
LAST DAY -- Gets the last day of the month for the given date.
LOCALTIME -- Gets the current local date and time.
LOCALTIMESTAMP -- Same as LOCALTIME.
MAKEDATE -- Creates a date and returns it, based on the given year and number of days values.
MAKETIME -- Creates a time and returns it, based on the given hour, minute and second values.
MICROSECOND -- Returns the microsecond of a given time or datetime.
MINUTE -- Returns the minute of the given time or datetime.
MONTH -- Returns the month of the given date.
MONTHNAME -- Returns the name of the month of the given date.
NOW -- Same as LOCALTIME.
PERIOD_ADD -- Adds the given number of months to the given period.
PERIOD_DIFF -- Returns the difference between 2 given periods.
QUARTER -- Returns the year quarter for the given date.
SECOND -- Returns the second of a given time or datetime.
SEC_TO_TIME -- Returns a time based on the given seconds.
STR_TO_DATE -- Creates a date and returns it based on the given string and format.
SUBDATE -- Same as DATE_SUB.
SUBTIME -- Subtracts a time interval (eg: 02:00) to a time or datetime (05:00) and returns the result (03:00).
SYSDATE -- Same as LOCALTIME.
TIME -- Returns the time from a given time or datetime.
TIME_FORMAT -- Returns the given time in the given format.
TIME_TO_SEC -- Converts and returns a time into seconds.
TIMEDIFF -- Returns the difference between 2 given time/datetime expressions.
TIMESTAMP -- Returns the datetime value of the given date or datetime.
TO_DAYS -- Returns the total number of days that have passed from ‘00-00-0000’ to the given date.
WEEK -- Returns the week number for the given date.
WEEKDAY -- Returns the weekday number for the given date.
WEEKOFYEAR -- Returns the week number for the given date.
YEAR -- Returns the year from the given date.
YEARWEEK -- Returns the year and week number for the given date. 4. Diverse funktioner i SQL
BIN -- Returns the given number in binary.
BINARY -- Returns the given value as a binary string.
CAST -- Converst one type into another.
COALESCE -- From a list of values, returns the first non-null value.
CONNECTION_ID -- For the current connection, returns the unique connection ID.
CONV -- Converts the given number from one numeric base system into another.
CONVERT -- Converts the given value into the given datatype or character set.
CURRENT_USER -- Returns the user and hostname which was used to authenticate with the server.
DATABASE -- Gets the name of the current database.
GROUP BY -- Used alongside aggregate functions (COUNT, MAX, MIN, SUM, AVG) to group the results.
HAVING -- Used in the place of WHERE with aggregate functions.
IF -- If the condition is true it returns a value, otherwise it returns another value.
IFNULL -- If the given expression equates to null, it returns the given value.
ISNULL -- If the expression is null, it returns 1, otherwise returns 0.
LAST_INSERT_ID -- For the last row which was added or updated in a table, returns the auto increment ID.
NULLIF -- Compares the 2 given expressions. If they are equal, NULL is returned, otherwise the first expression is returned.
SESSION_USER -- Returns the current user and hostnames.
SYSTEM_USER -- Same as SESSION_USER.
USER -- Same as SESSION_USER.
VERSION -- Returns the current version of the MySQL powering the database. Jokertegn i SQL
I SQL er jokertegn specialtegn, der bruges sammen med LIKE og NOT LIKE søgeord. Dette giver os mulighed for at søge efter data med sofistikerede mønstre ret effektivt.
% -- Equates to zero or more characters.
-- Example: Find all customers with surnames ending in ‘ory’.
SELECT * FROM customers
WHERE surname LIKE '%ory';
_ -- Equates to any single character.
-- Example: Find all customers living in cities beginning with any 3 characters, followed by ‘vale’.
SELECT * FROM customers
WHERE city LIKE '_ _ _vale';
[charlist] -- Equates to any single character in the list.
-- Example: Find all customers with first names beginning with J, K or T.
SELECT * FROM customers
WHERE first_name LIKE '[jkt]%'; SQL-nøgler
I relationelle databaser er der et koncept om primær og udenlandsk nøgler. I SQL-tabeller er disse inkluderet som begrænsninger, hvor en tabel kan have en primærnøgle, en fremmednøgle eller begge dele.
1. Primære nøgler i SQL
En primær lader hver post i en tabel identificeres entydigt. Du kan kun have én primær nøgle pr. tabel, og du kan tildele denne begrænsning til enhver enkelt eller kombination af kolonner. Dette betyder dog, at hver værdi i denne eller disse kolonner skal være unikke.
Typisk i en tabel er ID-kolonnen en primær nøgle og er normalt parret med AUTO_INCREMENT søgeord. Dette betyder, at værdien stiger automatisk, når og når nye poster oprettes.
Eksempel (MySQL)
Opret en ny tabel, og indstil den primære nøgle til ID-kolonnen.
CREATE TABLE customers (
id int NOT NULL AUTO_INCREMENT,
FirstName varchar(255),
Last Name varchar(255) NOT NULL,
address varchar(255),
email varchar(255),
PRIMARY KEY (id)
); 2. Fremmednøgler i SQL
Du kan anvende en fremmednøgle til én eller flere kolonner. Du bruger det til at linke 2 tabeller sammen i en relationsdatabase.
Tabellen, der indeholder fremmednøglen, kaldes underordnet nøgle,
Tabellen, der indeholder den refererede (eller kandidat) nøgle kaldes overordnet tabel.
Dette betyder i bund og grund, at kolonnedataene deles mellem 2 tabeller, fordi en fremmednøgle også forhindrer ugyldige data i at blive indsat, som ikke også er til stede i den overordnede tabel.
Eksempel (MySQL)
Opret en ny tabel og forvandl enhver kolonne, der refererer til ID'er i andre tabeller, til fremmednøgler.
CREATE TABLE orders (
id int NOT NULL,
user_id int,
product_id int,
PRIMARY KEY (id),
FOREIGN KEY (user_id) REFERENCES users(id),
FOREIGN KEY (product_id) REFERENCES products(id)
); Indekser i SQL
Indekser er attributter, der kan tildeles til kolonner, som der ofte søges efter for at gøre datahentning til en hurtigere og mere effektiv proces.
CREATE INDEX -- Creates an index named ‘idx_test’ on the first_name and surname columns of the users table. In this instance, duplicate values are allowed.
CREATE INDEX idx_test
ON users (first_name, surname);
CREATE UNIQUE INDEX -- The same as the above, but no duplicate values.
CREATE UNIQUE INDEX idx_test
ON users (first_name, surname);
DROP INDEX -- Removes an index.
ALTER TABLE users
DROP INDEX idx_test; SQL-joins
I SQL er en JOIN klausul bruges til at returnere et resultat, der kombinerer data fra flere tabeller, baseret på en fælles kolonne, som er vist i dem begge.
Der er en række forskellige joins tilgængelige for dig at bruge:
- Indre tilslutning (standard): Returnerer alle poster, der har matchende værdier i begge tabeller.
- Venstre deltagelse: Returnerer alle posterne fra den første tabel sammen med eventuelle matchende poster fra den anden tabel.
- Tilslut til højre: Returnerer alle posterne fra den anden tabel sammen med eventuelle matchende poster fra den første.
- Fuld tilmelding: Returnerer alle poster fra begge tabeller, når der er et match.
En almindelig måde at visualisere, hvordan joinforbindelser fungerer, er som denne:
SELECT orders.id, users.FirstName, users.Surname, products.name as ‘product name’
FROM orders
INNER JOIN users on orders.user_id = users.id
INNER JOIN products on orders.product_id = products.id; Visninger i SQL
En visning er i bund og grund et SQL-resultatsæt, der bliver gemt i databasen under en etiket, så du kan vende tilbage til det senere uden at skulle køre forespørgslen igen.
Disse er især nyttige, når du har en dyr SQL-forespørgsel, som du muligvis har brug for flere gange. Så i stedet for at køre det igen og igen for at generere det samme resultatsæt, kan du bare gøre det én gang og gemme det som en visning.
Sådan opretter du visninger i SQL
For at oprette en visning kan du gøre det sådan her:
CREATE VIEW priority_users AS
SELECT * FROM users
WHERE country = ‘United Kingdom’; Hvis du så fremover har brug for at få adgang til det gemte resultatsæt, kan du gøre det sådan her:
SELECT * FROM [priority_users]; Sådan erstattes visninger i SQL
Med CREATE OR REPLACE kommando, kan du opdatere en visning som denne:
CREATE OR REPLACE VIEW [priority_users] AS
SELECT * FROM users
WHERE country = ‘United Kingdom’ OR country=’USA’; Sådan sletter du visninger i SQL
For at slette en visning skal du blot bruge DROP VIEW kommando.
DROP VIEW priority_users; Konklusion
De fleste websteder og applikationer bruger relationelle databaser på en eller anden måde. Dette gør SQL ekstremt værdifuldt at kende, da det giver dig mulighed for at skabe mere komplekse, funktionelle systemer.
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