【Codecademy】Learn SQL 评价
本来啊，是想点评一下这门课晒晒“证书”的。结果在MOOC学院找不到codecademy开的这门课，还发现MOOC学院 codecademy 的课程页面有bug…
而且 Course 页面机构的选项，直接没有 codecademy 吗？！！
>>>>> 接下来谈论这门课 Learn SQL
评价
知识量5，趣味性4，教师参与0，课程设计5，难度=简单，进度已完成
这门课知识/语法讲的很清楚啊，章节也很合理，花3个晚上每次看两个小时就能解决！！一下子就明白了一个月以来，我们老师在课堂上扯了什么鬼。
课程内容
1.
MANIPULATION
2.
QUERIES
3.
AGGREGATE FUNCTIONS
4.
MULTIPLE TABLES
这就是总结啦，跟着学习没问题的！能够直接查询表格、比较结果和猜想，比起老师上课的例子、习题好多了（只能脑补，只有自己的判断而没有结果对比）。不过呢，学完一遍也只是知道了这些语法的用法，而且有些现在已经记不得了，还是需要多实践来熟练使用。
最后附文字版
Congratulations! You’ve learned six commands commonly used to manage
data stored in a relational database. What can we generalize so far?
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.
UPDATE edits a row in a table.
ALTER TABLE changes an existing table.
DELETE FROM deletes rows from a table.
Congratulations! You just learned how to query data from a database
using SQL. What can we generalize so far?
SELECT is the clause you use every time you want to query information
from a database.
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 that can be used in a WHERE
clause
AND and OR are special operators that you can use with WHERE to filter
the query on two or more conditions.
ORDER BY lets you sort the results of the query in either ascending or
descending order.
LIMIT lets you specify the maximum number of rows that the query will
return. This is especially important in large tables that have thousands
or even millions of rows.
Congratulations! You just learned how to use aggregate functions to
perform calculations on your data. What can we generalize so far?
Aggregate functions combine multiple rows together to form a single
value of more meaningful information.
COUNT takes the name of a column(s) as an argument and counts the number
of rows where the value(s) is not NULL.
GROUP BY is a clause used with aggregate functions to combine data from
one or more columns.
SUM() takes the column name as an argument and returns the sum of all
the values in that column.
MAX() takes the column name as an argument and returns the largest value
in that column.
MIN() takes the column name as an argument and returns the smallest
value in that column.
AVG() takes a column name as an argument and returns the average value
for that column.
ROUND() takes two arguments, a column name and the number of decimal
places to round the values in that column.
Congratulations! You just learned how to query information from multiple
tables using SQL. What can we generalize so far?
Primary Key is a column that serves a unique identifier for row in the
table. Values in this column must be unique and cannot be NULL.
Foreign Key is a column that contains the primary key to another table
in the database. It is used to identify a particular row in the
referenced table.
Joins are used in SQL to combine data from multiple tables.
INNER JOIN will combine rows from different tables if the join condition
is true.
LEFT OUTER 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.
AS is a keyword in SQL that allows you to rename a column or table in
the result set using an alias.
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