Teaching Kids Programming: Videos on Data Structures and Algorithms
The SQL (Structured Query Language) is a programming language that is used to interact with the Database. The SELECT statement is used to retrieve (and filter) data from Database (which contains the Organised data).
A Database con contain many tables. A table in a RDMS (Relational Database Management System) contains rows and columns. The columns are attributes (meta data), the rows are the actual data.
Given the following table named Person
Name | Age | Gender | Score --------------------------- Eric | 10 | Male | 90 --------------------------- Ryan | 8 | Male | 85 --------------------------- Yan | 9 | Female | 87 ---------------------------
We have columns “Name”, “Age”, “Gender” and “Score”. We can add indexes to columns. The indices are used to speed up searches. If there is no indices or primary keys of a table, the database has to go through each row to find the matched records.
A table can only have a primary key which is a unique index.
The basic SELECT statement is:
SELECT something FROM a_table WHERE some_conditions ORDER BY some_columns
The where and order by clause are optional.
To select everything, we use “*” (wild chars or asterisk). We can select multiple columns, separated by “,”
We can select from multiple tables (we can join the tables by some common columns).
The where clause is to specify the filter criteria.
We can apply aggregate functions such as max, min, avg, sum on the columns.
The order-by clause can specify one or more columns for results to be sorted. By Default, the order is ascending (ASC), and we can specify the DESC to sort the result in descending order.
Here are a few examples of SQL:
SQL to Get the name and score of person who is age 10:
SELECT Name, Score FROM Persons WHERE Age = 10
SQL to Get the max age:
SELECT Max(Age) FROM Persons
SQL to Sort the Table by Score Descending order:
SELECT * FROM Persons ORDER BY Score DESC
Learning SQL
- Teaching Kids Programming - Second Highest Distinct Record using SQL and Python
- Teaching Kids Programming - Most Common SQL keywords (Select, Update, Insert, Delete)
- Teaching Kids Programming - Introduction to SQL and the SELECT
- How to Match Word Boundary using SQL?
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