Introduction to SQL Commands
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In today’s world of increased digitization, big data, and cloud computing, data management is amongst the most important skills a software engineer can have. To this end, one of the most powerful database tools is SQL.
SQL (Structured Query Language) is the standard programming language used to manipulate data structure objects. They operate upon data that is contained in a relational database management system (RDBMS). Some well-known RDBMS are MySQL and PostgreSQL.
In this guide, you learn about the subsets of the SQL language and how to use some fundamental SQL commands, like
Subsets of SQL
The list below includes the different language subsets of various SQL commands. Each subset has its own function and purpose.
- Data Definition Language (DDL): This allows you to create, delete, and update database schema definitions (namely, tables and indexes), without actually manipulating the data within the database tables.
- Data Query Language (DQL): DQL is used to retrieve data from the database using the
- Data Manipulation Language (DML): This sublanguage allows for data manipulation in the database using the
This guide uses an example database for a school to further demonstrate the SQL commands for each subset listed above. The school database has several tables, for students, courses, grades, and so forth. The definition of the
Student table contains columns for the student’s
Lastname, and the definition of the
CourseTaken table contains columns for
The example assumes that there are three students in the school, each of which has completed two courses. The sample data is shown in the table below:
Create, Alter, and Drop Tables using SQL Commands
From the command line, use the
CREATE TABLE command followed by the name of the table and the table data. The command below creates the
CREATE TABLE Student ( SSNumber CHAR(9) NOT NULL, LastName VARCHAR(30) NOT NULL, FirstName VARCHAR(20) NOT NULL );
The parenthesis encloses the table data, starting with a column that labels each row’s data. The next column indicates the data type that this row holds.
CHAR indicates a fixed-length string data type and
VARCHAR indicates a variable-length string data type. In the final column, the
NOT NULL attribute ensures that a record cannot be added to the table if any of the
NOT NULL columns do not have data associated with them.
CREATE TABLEstatement is delimited with a trailing semicolon (;), although it is possible that some commercial relational database systems may not require that delimiter.
NoteUnless mentioned otherwise, all the database commands demonstrated in this guide work well on both MySQL and PostgreSQL.
To create the
CourseTaken table, execute the following command:
CREATE TABLE CourseTaken ( SSNumber CHAR(9) NOT NULL, CourseId CHAR(6) NOT NULL, NumericGrade INT NOT NULL );
YearTaken column is intentionally not included in the
CourseTaken table to demonstrate the usage of the
ALTER TABLE command. To add the
YearTaken column in the
CourseTaken table, you don’t need to drop the
CourseTaken table entirely. Instead, you can use the DDL
ALTER TABLE command. The following command alters the
CourseTaken table by adding the missing column to the table.
ALTER TABLE CourseTaken ADD (YearTaken INT NOT NULL);
The command above follows a similar syntax as before. It requires the table name as well as three arguments: row name, row data type, and
NOT NULL attribute. If you want to delete the
CourseTaken table entirely, issue the DDL
DROP TABLE command followed by the table name.
DROP TABLE CourseTaken;
CautionDropping a table deletes all the data in the table.
How to Insert Data Into a Table in SQL
To insert the data into the table, use the SQL
INSERT INTO statement. To call this command, provide the table name and the list of row names (in parenthesis) that you want to insert the data into. This is followed by the
VALUES keyword and the actual values (in parenthesis) that you wish to insert. The values are inserted into the rows in order of which they are called.
- SQL commands can be broken up across lines. The end of the SQL command is delimited by a semicolon (
- The character data is delimited by an opening and closing apostrophe (
‘), whereas numeric data is not.
INSERT commands insert three rows into the
Student table. These commands use multiple
INSERT INTO Student (SSNumber, LastName, FirstName) VALUES ('111111111', 'Smith', 'John'); INSERT INTO Student (SSNumber, LastName, FirstName) VALUES ('222222222', 'Jones', 'Mary'); INSERT INTO Student (SSNumber, LastName, FirstName) VALUES ('333333333', 'Hansen', 'Robert');
Similarly, you can also insert multiple rows into the table in a single SQL query as shown below:
INSERT INTO CourseTaken (SSNumber, CourseId, NumericGrade, YearTaken) VALUES ('111111111', 'CSC101', 98, 2021), ('111111111', 'ENG101', 95, 2022), ('222222222', 'CSC101', 100, 2022);
You can use the
INSERT INTOcommand similarly in PostgreSQL to add rows to the table. Make sure the values match the order of the columns in the table definition.
INSERT INTO student VALUES ('111111111', 'Smith', 'John');
Delete Data From a Table
To delete data from a table, use the SQL
DELETE FROM statement. Use the
WHERE clause to specify the condition, and if there is more than one condition, use the
AND clause along with
For example, the following command deletes a record from the
CourseTaken table with SSNumber
333333333 and CourseId
CautionIf you omit the
WHEREclause, all the records in the table are deleted.
DELETE FROM CourseTaken WHERE SSNumber = '333333333' AND CourseId = 'POL101';
SQL Command to Update Data in a Table
To update the existing record in a table, use the SQL
UPDATE command. The
SET clause is used to set (update) a new value to a particular column and the
WHERE clause is used to update the selected rows.
For example, the following command updates the
NumericGrade column of the
CourseTaken table for records with SSNumber
222222222 and CourseId
UPDATE CourseTaken SET NumericGrade = 95 WHERE SSNumber = '222222222' AND CourseId = 'EEE101';
SQL Command to Retrieve Data From a Table
The true power of relational database systems is in its ability to retrieve information in a multi-table schema, via the SQL
SELECT command, and the ability to join tables via common keys. Although this introductory guide does not examine the creation of keys and indexes utilizing those keys, it utilizes the
SSNumber column of each table as a vehicle (key) to relate (or join) the tables to generate information. The following examples provide different use cases of using the SQL
SELECT command from the command line.
Example 1: To fetch the list of all students in the school.
SELECT * from Student;
+-----------+----------+-----------+ | SSNumber | LastName | FirstName | +-----------+----------+-----------+ | 111111111 | Smith | John | | 222222222 | Jones | Mary | | 333333333 | Hansen | Robert | +-----------+----------+-----------+
Example 2: To fetch the list of all students and courses they have taken.
SELECT Student.SSNumber, Student.LastName, Student.FirstName, CourseTaken.CourseId FROM Student, CourseTaken WHERE Student.SSNumber = CourseTaken.SSNumber;
+-----------+----------+-----------+----------+ | SSNumber | LastName | FirstName | CourseId | +-----------+----------+-----------+----------+ | 111111111 | Smith | John | CSC101 | | 111111111 | Smith | John | ENG101 | | 222222222 | Jones | Mary | CSC101 | +-----------+----------+-----------+----------+
NoteIn the above command, the two tables,
CourseTakenare joined to retrieve the required information. The column names in the
WHEREclauses are prefixed with their table names for clarity. However, in the case of the
SSNumbercolumn, we are required to specify the appropriate table name prefixes, since both tables share the same column name. The
FROMclause indicates the tables that are being used in this query.
Example 3: Retrieve the list of students with CourseId
CSC101 and the year that they took this course.
SELECT Student.LastName, Student.FirstName, CourseTaken.CourseId, CourseTaken.YearTaken FROM Student, CourseTaken WHERE Student.SSNumber = CourseTaken.SSNumber AND CourseTaken.CourseId = 'CSC101';
+----------+-----------+----------+-----------+ | LastName | FirstName | CourseId | YearTaken | +----------+-----------+----------+-----------+ | Smith | John | CSC101 | 2021 | | Jones | Mary | CSC101 | 2022 | +----------+-----------+----------+-----------+
Example 4: Retrieve the list of student names, courses taken and grades received, for those that had course grades above
SELECT Student.LastName, Student.FirstName, CourseTaken.CourseId, CourseTaken.NumericGrade FROM Student, CourseTaken WHERE Student.SSNumber = CourseTaken.SSNumber AND CourseTaken.NumericGrade > 90;
+----------+-----------+----------+--------------+ | LastName | FirstName | CourseId | NumericGrade | +----------+-----------+----------+--------------+ | Smith | John | ENG101 | 95 | | Smith | John | CSC101 | 98 | | Jones | Mary | CSC101 | 100 | +----------+-----------+----------+--------------+
ANDclause in the command above allows you to filter the results by a conditional grade score test.
This guide on SQL commands is an introductory primer on how to create database schemas and manipulate data within those databases. Although the concepts introduced here merely scratch the surface in regard to relational database systems’ usage, it as a good starting point for basic and essential commands and concepts.
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