Check out example codes for "learnxinyminutes sql". It will help you in understanding the concepts better.
Code Example 1
-- Comments start with two hyphens. End each command with a semicolon. -- SQL is not case-sensitive about keywords. The sample commands here -- follow the convention of spelling them in upper-case because it makes -- it easier to distinguish them from database, table, and column names. -- Create and delete a database. Database and table names are case-sensitive. CREATE DATABASE someDatabase; DROP DATABASE someDatabase; -- List available databases. SHOW DATABASES; -- Use a particular existing database. USE employees; -- Select all rows and columns from the current database's departments table. -- Default activity is for the interpreter to scroll the results on your screen. SELECT * FROM departments; -- Retrieve all rows from the departments table, -- but only the dept_no and dept_name columns. -- Splitting up commands across lines is OK. SELECT dept_no, dept_name FROM departments; -- Retrieve all departments columns, but just 5 rows. SELECT * FROM departments LIMIT 5; -- Retrieve dept_name column values from the departments -- table where the dept_name value has the substring 'en'. SELECT dept_name FROM departments WHERE dept_name LIKE '%en%'; -- Retrieve all columns from the departments table where the dept_name -- column starts with an 'S' and has exactly 4 characters after it. SELECT * FROM departments WHERE dept_name LIKE 'S____'; -- Select title values from the titles table but don't show duplicates. SELECT DISTINCT title FROM titles; -- Same as above, but sorted (case-sensitive) by the title values. SELECT DISTINCT title FROM titles ORDER BY title; -- Show the number of rows in the departments table. SELECT COUNT(*) FROM departments; -- Show the number of rows in the departments table that -- have 'en' as a substring of the dept_name value. SELECT COUNT(*) FROM departments WHERE dept_name LIKE '%en%'; -- A JOIN of information from multiple tables: the titles table shows -- who had what job titles, by their employee numbers, from what -- date to what date. Retrieve this information, but instead of the -- employee number, use the employee number as a cross-reference to -- the employees table to get each employee's first and last name -- instead. (And only get 10 rows.) SELECT employees.first_name, employees.last_name, titles.title, titles.from_date, titles.to_date FROM titles INNER JOIN employees ON employees.emp_no = titles.emp_no LIMIT 10; -- List all the tables in all the databases. Implementations typically provide -- their own shortcut command to do this with the database currently in use. SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_TYPE='BASE TABLE'; -- Create a table called tablename1, with the two columns shown, for -- the database currently in use. Lots of other options are available -- for how you specify the columns, such as their datatypes. CREATE TABLE tablename1 (fname VARCHAR(20), lname VARCHAR(20)); -- Insert a row of data into the table tablename1. This assumes that the -- table has been defined to accept these values as appropriate for it. INSERT INTO tablename1 VALUES('Richard','Mutt'); -- In tablename1, change the fname value to 'John' -- for all rows that have an lname value of 'Mutt'. UPDATE tablename1 SET fname='John' WHERE lname='Mutt'; -- Delete rows from the tablename1 table -- where the lname value begins with 'M'. DELETE FROM tablename1 WHERE lname like 'M%'; -- Delete all rows from the tablename1 table, leaving the empty table. DELETE FROM tablename1; -- Remove the entire tablename1 table. DROP TABLE tablename1;
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