IT

String Functions ASCII Return the ASCII value for the specific character CHAR_LENGTH or CHARACTER_LENGTH Return the length of string (in characters) CONCAT Adds two or more expressions together CONCAT_WS Adds two or more expressions together with a separator FIELD Returns the index position of a value in a list of values FIND_IN_SET Returns the position of a string within a list of strings FORMAT Return sFormats a number to a format like “#,###,###.##”, rounded to a specified number of decimal places INSERT Inserts a string within a string at the specified position and for a certain number of characters INSTR Returns the position of the first occurrence of a string in another string LCASE Converts a string to lower-case LEFT Extracts a number of characters from a string (starting from left) LENGTH Returns the length of a string (in bytes) LOCATE Returns the position of the first occurrence of a substring in a string LOWER Converts a string to lower-case LPAD Left-pads a string with another string, to a certain length LTRIM Removes leading spaces from a string MID Extracts a substring from a string (starting at any position) POSITION Returns the position of the first occurrence of a substring in a string REPEAT Repeats a string as many times as specified REPLACE Replaces all occurrences of a substring within a string, with a new substring REVERSE Reverses a string and returns the result RIGHT Extracts a number of characters from a string (starting from right) RPAD Right-pads a string with another string, to a certain length RTRIM Removes trailing spaces from a string SPACE Returns a string of the specified number of space characters STRCMP Compares two strings SUBSTR Extracts a substring from a string (starting at any position) SUBSTRING Extracts a substring from a string (starting at any position) SUBSTRING_INDEX Returns a substring of a string before a specified number of delimiter occurs TRIM Removes leading and trailing spaces from a string UCASE Converts a string to upper-case UPPER Converts a string to upper-case Numeric Functions ABS Returns the absolute value of a number ACOS Returns the arc cosine of a number ASIN Returns the arc sine of a number ATAN Returns the arc tangent of one or two numbers ATAN2 Returns the arc tangent of two numbers AVG Returns the average value of an expression CEIL Returns the smallest integer value that is >= to a number CEILING Returns the smallest integer value that is >= to a number COS Returns the cosine of a number COT Returns the cotangent of a number COUNT Returns the number of records returned by a select query DEGREES Converts a value in radians to degrees DIV Used for integer division EXP Returns e raised to the power of a specified number FLOOR Returns the largest integer value that is <= to a number GREATEST Returns the greatest value of the list of arguments LEAST Returns the smallest value of the list of arguments LN Returns the natural logarithm of a number LOG Returns the natural logarithm of a number, or the logarithm of a number to a specified base LOG10 Returns the natural logarithm of a number to base 10 LOG2 Returns the natural logarithm of a number to base 2 MAX Returns the maximum value in a set of values MIN Returns the minimum value in a set of values MOD Returns the remainder of a number divided by another number PI Returns the value of PI POW Returns the value of a number raised to the power of another number POWER Returns the value of a number raised to the power of another number RADIANS Converts a degree value into radians RAND Returns a random number ROUND Rounds a number to a specified number of decimal places SIGN Returns the sign of a number SIN Returns the sine of a number SQRT Returns the square root of a number SUM Calculates the sum of a set of values TAN Returns the tangent of a number TRUNCATE Truncates a number to the specified number of decimal places Date Functions ADDDATE Adds a time/date interval to a date and then returns the date ADDTIME Adds a time interval to a time/datetime and then returns the time/datetime CURDATE Returns the current date CURRENT_DATE Returns the current date CURRENT_TIME Returns the current time CURRENT_TIMESTAMP Returns the current date and time CURTIME Returns the current time DATE Extracts the date part from a datetime expression DATEDIFF Returns the number of days between two date values DATE_ADD Adds a time/date interval to a date and then returns the date DATE_FORMAT Formats a date DATE_SUB Subtracts a time/date interval from a date and then returns the date DAY Returns the day of the month for a given date DAYNAME Returns the weekday name for a given date DAYOFMONTH Returns the day of the month for a given date DAYOFWEEK Returns the weekday index for a given date DAYOFYEAR Returns the day of the year for a given date EXTRACT Extracts a part from a given date FROM_DAYS Returns a date from a numeric datevalue HOUR Returns the hour part for a given date LAST_DAY Extracts the last day of the month for a given date LOCALTIME Returns the current date and time LOCALTIMESTAMP Returns the current date and time MAKEDATE Creates and returns a date based on a year and a number of days value MAKETIME Creates and returns a time based on an hour, minute, and second value MICROSECOND Returns the microsecond part of a time/datetime MINUTE Returns the minute part of a time/datetime MONTH Returns the month part for a given date MONTHNAME Returns the name of the month for a given date NOW Returns the current date and time PERIOD_ADD Adds a specified number of months to a period PERIOD_DIFF Returns the difference between two periods QUARTER Returns the quarter of the year for a given date value SECOND Returns the seconds part of a time/datetime SEC_TO_TIME Returns a time value based on the specified seconds STR_TO_DATE Returns a date based on a string and a format SUBDATE Subtracts a time/date interval from a date and then returns the date SUBTIME Subtracts a time interval from a datetime and then returns the time/datetime SYSDATE Returns the current date and time TIME Extracts the time part from a given time/datetime TIME_FORMAT Formats a time by a specified format TIME_TO_SEC Converts a time value into seconds TIMEDIFF Returns the difference between two time/datetime expressions TIMESTAMP Returns a datetime value based on a date or datetime value TO_DAYS Returns the number of days between a date and date “0000-00-00” WEEK Returns the week number for a given date WEEKDAY Returns the weekday number for a given date WEEKOFYEAR Returns the week number for a given date YEAR Returns the year part for a given date YEARWEEK Returns the year and week number for a given date Advanced Functions BIN Returns a binary representation of a number BINARY Converts a value to a binary string CASE Goes through conditions and return a value when the first condition is met CAST Converts a value (of any type) into a specified datatype COALESCE Returns the first non-null value in a list CONNECTION_ID Returns the unique connection ID for the current connection CONV Converts a number from one numeric base system to another CONVERT Converts a value into the specified datatype or character set CURRENT_USER Returns the user name and host name for the MySQL account that the server used to authenticate the current client DATABASE Returns the name of the current database IF Returns a value if a condition is TRUE, or another value if a condition is FALSE IFNULL Return a specified value if the expression is NULL, otherwise return the expression ISNULL Returns 1 or 0 depending on whether an expression is NULL LAST_INSERT_ID Returns the AUTO_INCREMENT id of the last row that has been inserted or updated in a table NULLIF Compares two expressions and returns NULL if they are equal. Otherwise, the first expression is returned SESSION_USER Returns the current MySQL user name and host name SYSTEM_USER Returns the current MySQL user name and host name USER Returns the current MySQL user name and host name VERSION Returns the current version of the MySQL database

Introduction What is SQL? SQL stands for Structured Query Language SQL lets you access and manipualate databases SQL becaome a standard of the American National Standards Institute (ANSI) in 1986, and of the International Organization for Standardization (ISO) in 1987 RDBMS RDBMS stands for Relational Database Management System. RDBMS is the basis for SQL, and for all modern database systems such as MS SQL Server, IBM DB2, Oracle, MySQL, and Microsoft Access. The data in RDBMS is stored in database objects called tables. A table is a collection of related data entries and it consists of columns and rows. Syntax SELECT * FROM Customers; SQL keywords are NOT case sensitive. Some database systems requires a semicolon at the end of each SQL statement. Semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server. Most Important SQL Commands SELECT - extracts data from a database UPDATE - updates data in a database DELETE - deletes data from a database INSERT INTO - inserts new data into a database CREATE DATABASE - creates a new database ALTER DATABASE - modifies a database CREATE TABLE - creates a new table ALTER TABLE - modifies a table DROP TABLE - deletes a table CREATE INDEX - creates an index (search key) DROP INDEX - deletes an index SELECT The SELECT statement is used to select data from a database. The data returned is stored in a result table, called the result-set. SELECT column1, column2, ... FROM table_name; SELECT DISTINCT The SELECT DINSTINCT statement is used to return only distinct (different) values. Inside a table, a column often contains many duplicate values; and sometimes you only want to list the different (distinct) values. SELECT DISTINCT column1, column2, ... FROM table_name; WHERE The WHERE clause is used to filter records. It is used to extract only those records that fultill a specified condition. SELECT column1, column2, ... FROM table_name WHERE condition; Operators can be used in the WHERE clause: \(\begin{array}{|c|l|}\hline \text{Operator}&\text{Description}\\\hline \text{=}&\text{Equal}\\\hline \text{>}&\text{Greater than}\\\hline \text{<}&\text{Less than}\\\hline \text{>=}&\text{Greater than or equal}\\\hline \text{<=}&\text{Less than or equal}\\\hline \text{<>, !=}&\text{Not equal}\\\hline \text{BETWEEN}&\text{Between a certain range}\\\hline \text{LIKE}&\text{Search for a pattern}\\\hline \text{IN}&\text{To specify multiple possible values for a column}\\\hline \end{array}\) AND, OR, NOT THE WHERE clause can be combined with AND, OR, and NOT operators. The AND and OR operators are used to filter records based on more than one condition: The AND operator displays a record if all the conditons separated by AND are TRUE. The OR operator displays a record if any of the conditons separated by OR is TRUE. THE NOT operator display a record if the condition(s) it NOT TRUE. SELECT column1, column2, ... FROM table_name; WHERE NOT condition1 AND condition2 OR condition3 ...; ORDER BY The ORDER BY keyword is used to sort the result-set in ascending or descending order. The ORDER BY keyword sorts the records in ascending order by default. To sort the records in descending order, used DESC keyword. SELECT column1, column2, ... FROM table_name ORDER BY column1, column2, ... ASC|DESC; INSERT INTO The INSERT INTO statement is used to insert new records in a table. Specify both the column names and the values to be inserted: INSERT INTO table_name (column1, column2, column3, ...) VALUES (value1, value2, value3, ...); If you are adding values for all the columns of the table, you do not need to specify the column names in the SQL query. However, make sure the order of the values is in the same order as the columns in the table. Here, the INSERT INTO syntax would be as follows: INSERT INTO table_name VALUES (value1, value2, value3, ...); IS NULL, IS NOT NULL What is a NULL Value? ...

Microservices Definition Separate business logic functions. Instead of one big problem, several smaller applications. Communicative via well defined APIs - usually HTTP In demand Advantages and Disavantages Advantages Language independent Fast iterations Small teams Fault Isolation Pair well with containers Scalable Big plus Disavantages Complex networking Overhead Databases Servers Docker is an open platform for developers and sysadmins to build, ship, and run distributed applications, whether on laptops, data center VMs, or the cloud. ...

0. 前言 以往寫筆記通常是使用 Notion，簡單的 Markdown 語法搭配支援 LaTex，使得在撰寫學習筆記時，可以達到快速且美觀的呈現。 雖說 GitHub Pages 支援了 Markdown 的語法，但卻不支援 LaTex，這使得想將筆記從 Notion 移轉到 GitHub Pages，成為一個小缺點。 而此處介紹的是在網頁上顯示較為輕便的 KaTex。 1. 解決方法 利用 KaTex，在靜態頁面掛載 JavaScript 程式碼。 其在官網的描述是： Beautilful math in all browsers A JavaScript display engine for mathematics that works in all browsers. No more setup for readers. It just works. Step 1. 創建一個可常駐的靜態頁面 在 \layouts\partials\ 下創建一個叫作 math.html 的頁面。 注意是 global 的 layouts 文件夾中不是 hugo themes 中的文件夾 開啟頁面之後，在 <head> 與 </head> 之間加入下一步驟內的 JavaScript 程式碼。 Step 2. 將 JavaScript 程式碼貼入 math.html <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/katex@0.13.18/dist/katex.min.css" integrity="sha384-zTROYFVGOfTw7JV7KUu8udsvW2fx4lWOsCEDqhBreBwlHI4ioVRtmIvEThzJHGET" crossorigin="anonymous"> <script defer src="https://cdn.jsdelivr.net/npm/katex@0.13.18/dist/katex.min.js" integrity="sha384-GxNFqL3r9uRJQhR+47eDxuPoNE7yLftQM8LcxzgS4HT73tp970WS/wV5p8UzCOmb" crossorigin="anonymous"></script> <script defer src="https://cdn.jsdelivr.net/npm/katex@0.13.18/dist/contrib/auto-render.min.js" integrity="sha384-vZTG03m+2yp6N6BNi5iM4rW4oIwk5DfcNdFfxkk9ZWpDriOkXX8voJBFrAO7MpVl" crossorigin="anonymous" onload="renderMathInElement(document.body);"></script> 上面的程式碼是將 KaTex 的渲染程式碼寫到 math.html 中。 Step 3. 將 Hugo theme 中的 header.html 複製出來 同樣將 header.html 複製到 global 下的 \layout\partials，並貼入 {{- /* Head custom content area start */ -}} {{- /* Insert any custom code (web-analytics, resources, etc.) - it will appear in the <head></head> section of every page. */ -}} {{- /* Can be overwritten by partial with the same name in the global layouts. */ -}} {{ if or .Params.math .Site.Params.math }} {{ partial "math.html" . }} {{ end }} {{- /* Head custom content area end */ -}} 上面的程式碼的功能將 math.html 嵌進 header.html 中，並以 config.yml 中的 .param.math 指令來控制。 Step 4. 開始撰寫 KaTex 一開始先將標頭的定義區將 math: true，即可開啟該頁面的 KaTex 渲染功能。 在 \\( 符號與 \\) 符號間進行 inline equation 插入。 或在 $ $ 符號與 $ $ 間進行 block equation 的插入。 LaTex 的語法可以參考https://www.cs.pu.edu.tw/~wckuo/doc/latex123/node11.html 參考來源： ...