• 2022年1月1日

SQL 筆記

SQL is a programming language designed to manipulate and manage data stored in relational databases.

  • A relational database is a database that organizes information into one or more tables.
  • A table is a collection of data organized into rows and columns.

A statement is a string of characters that the database recognizes as a valid command.

  • CREATE TABLE creates a new table.
  • INSERT INTO adds a new row to a table.
  • SELECT queries data from a table.
  • ALTER TABLE changes an existing table.
  • UPDATE edits a row in a table.
  • DELETE FROM deletes rows from a table.

Constraints add information about how a column can be used.

  • SELECT is the clause we use every time we want to query information from a database.
  • AS renames a column or table.
  • DISTINCT return unique values.
  • WHERE is a popular command that lets you filter the results of the query based on conditions that you specify.
  • LIKE and BETWEEN are special operators.
  • AND and OR combines multiple conditions.
  • ORDER BY sorts the result.
  • LIMIT specifies the maximum number of rows that the query will return.
  • CASE creates different outputs.

  • COUNT(): count the number of rows
  • SUM(): the sum of the values in a column
  • MAX()/MIN(): the largest/smallest value
  • AVG(): the average of the values in a column
  • ROUND(): round the values in the column

Aggregate functions combine multiple rows together to form a single value of more meaningful information.

  • GROUP BY is a clause used with aggregate functions to combine data from one or more columns.
  • HAVING limit the results of a query based on an aggregate property.

  • JOIN will combine rows from different tables if the join condition is true.
  • LEFT JOIN will return every row in the left table, and if the join condition is not met, NULL values are used to fill in the columns from the right table.
  • Primary key is a column that serves a unique identifier for the rows in the table.
  • Foreign key is a column that contains the primary key to another table.
  • CROSS JOIN lets us combine all rows of one table with all rows of another table.
  • UNION stacks one dataset on top of another.
  • WITH allows us to define one or more temporary tables that can be used in the final query.

https://www.codecademy.com/learn/learn-sql/modules/learn-sql-manipulation/cheatsheet

comments powered by Disqus