SQLコマンドのチートシート–10分でSQLを学ぶ方法

コメントを使用すると、直接実行せずにSQLステートメントのセクションを説明できます。

SQLには、1行と複数行の2種類のコメントがあります。

SQLの単一行コメント

1行のコメントは「--」で始まります。これらの2文字から行末までのテキストはすべて無視されます。

-- This part is ignored

SELECT * FROM customers;

SQLの複数行コメント

複数行のコメントは/*で始まり、*/で終わります。終了文字が見つかるまで、複数の行にまたがって伸びます。

/*

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;

*/

MySQLのデータ型

新しいテーブルを作成するとき、または既存のテーブルを編集するときは、各列が受け入れるデータのタイプを指定する必要があります。

この例では、idに渡されるデータ 列はint（整数）である必要がありますが、FirstName 列にVARCHARがあります 最大255文字のデータ型。

CREATE TABLE customers(
id int,
FirstName varchar(255)
);

1。文字列データ型

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。数値データ型

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。日付/時刻データ型

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オペレーター

1。 SQLの算術演算子

+ -- Add
– -- Subtract
* -- Multiply
/ -- Divide
% -- Modulus

2。 SQLのビット演算子

& -- Bitwise AND
| -- Bitwise OR
^-- Bitwise XOR

3。 SQLの比較演算子

= -- Equal to
> -- Greater than
< -- Less than
>= -- Greater than or equal to
<= -- Less than or equal to
<> -- Not equal to

4。 SQLの複合演算子

+= -- Add equals
-= -- Subtract equals
*= -- Multiply equals
/= -- Divide equals
%= -- Modulo equals
&= -- Bitwise AND equals
^-= -- Bitwise exclusive equals
|*= -- Bitwise OR equals

SQL関数

1。 SQLの文字列関数

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。 SQLの数値関数

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。 SQLの日付関数

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。 SQLのその他の関数

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.

SQLのワイルドカード文字

SQLでは、ワイルドカードはLIKEで使用される特殊文字です。 およびNOT LIKE キーワード。これにより、高度なパターンのデータをかなり効率的に検索できます。

% -- 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キー

リレーショナルデータベースには、プライマリの概念があります。 および外国 キー。 SQLテーブルでは、これらは制約として含まれ、テーブルには主キー、外部キー、またはその両方を含めることができます。

1。 SQLの主キー

プライマリを使用すると、テーブル内の各レコードを一意に識別できます。テーブルごとに持つことができる主キーは1つだけであり、この制約を任意の単一または列の組み合わせに割り当てることができます。ただし、これは、この列内の各値が一意である必要があることを意味します。

通常、テーブルでは、ID列が主キーであり、通常はAUTO_INCREMENTとペアになっています。 キーワード。これは、新しいレコードが作成されると、値が自動的に増加することを意味します。

例（MySQL）

新しいテーブルを作成し、主キーをID列に設定します。

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。 SQLの外部キー

1つまたは複数の列に外部キーを適用できます。 リンクに使用します リレーショナルデータベースに2つのテーブルをまとめます。

外部キーを含むテーブルは、子と呼ばれます。 キー、

参照（または候補）キーを含むテーブルは、親と呼ばれます。 テーブル。

これは基本的に、列データが2つのテーブル間で共有されることを意味します。これは、外部キーによって、親テーブルにも存在しない無効なデータが挿入されるのを防ぐためです。

例（MySQL）

新しいテーブルを作成し、他のテーブルのIDを参照する列を外部キーに変換します。

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)
);

SQLのインデックス

インデックスは、データの取得をより迅速かつ効率的にするために頻繁に検索される列に割り当てることができる属性です。

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結合

SQLでは、JOIN 句は、両方のテーブルに含まれる共通の列に基づいて、複数のテーブルのデータを組み合わせた結果を返すために使用されます。

使用できるさまざまな結合があります：

• 内部結合（デフォルト）： 両方のテーブルで値が一致するレコードを返します。
• 左参加： 最初のテーブルのすべてのレコードと、2番目のテーブルの一致するレコードを返します。
• 右参加： 2番目のテーブルのすべてのレコードと、最初のテーブルの一致するレコードを返します。
• 完全参加： 一致する場合は、両方のテーブルからすべてのレコードを返します。

結合がどのように機能するかを視覚化する一般的な方法は次のとおりです。

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;

CREATE VIEW priority_users AS
SELECT * FROM users
WHERE country = ‘United Kingdom’;

SELECT * FROM [priority_users];

CREATE OR REPLACE VIEW [priority_users] AS
SELECT * FROM users
WHERE country = ‘United Kingdom’ OR country=’USA’;

DROP VIEW priority_users;