Adventures in Machine Learning

Mastering Data Retrieval Filtering Joining and Modification in Databases

The Fundamentals of Retrieving and Filtering Data

Data is a crucial aspect of any organization or project. Retrieving data is necessary to analyze, evaluate, and make informed decisions.

In this article, we will explore the primary keywords and techniques used for retrieving and filtering data from a database.

Retrieving all data from a table

To retrieve data from a table, we use the SELECT keyword. To retrieve all data, we can use the asterisk (*) symbol, followed by the FROM keyword, and then the name of the table.

The SELECT * FROM table_name query will return all the data present in the table.

Retrieving data from certain columns

Suppose we only require specific columns instead of the entire table. In that case, we can use the SELECT keyword, followed by the desired column names separated by commas, and then the FROM keyword and table name.

For instance, SELECT column1, column2 FROM table_name will retrieve only column1 and column2 data from the table.

Filtering data using WHERE clause with one condition

The WHERE clause helps in filtering data based on specific conditions. For instance, SELECT column_name FROM table_name WHERE column_name= value will retrieve data from the table where the column_name equals the given value.

This query will display all records where the column_name matches the given value.

Filtering data using AND operator for multiple conditions

Suppose we need to filter data based on multiple conditions that need to be met. In that case, we can use the AND operator to join two or more WHERE clauses together.

For instance, SELECT column_name FROM table_name WHERE column1= value1 AND column2=value2 will retrieve data where both column1 matches value1 and column2 matches value2- in the same record.

Filtering data using OR operator for multiple conditions

The OR operator works the same way as the AND operator. However, it returns records where at least one of the conditions is met.

For instance, SELECT column_name FROM table_name WHERE column_name=value1 OR column_name=value2 will retrieve data where the columnname matches either value 1 or value 2.

Using DISTINCT to retrieve non-repeated records

Suppose we have data with duplicates or replicas from the same records. In that case, executing a query to retrieve all the data in the table will include those replicas.

However, to eliminate those duplicates, we can use the DISTINCT keyword followed by the column names. For example, SELECT DISTINCT column_name FROM table_name will display only unique values of the column_name from the table.

Sorting and Searching Data

Sorting and searching data is essential when dealing with large datasets. The ORDER BY clause helps in arranging the retrieved data in a specific order.

Sorting data according to one column

The ORDER BY clause can be used to sort data based on a specific column in ascending or descending order. To do this, we can use the ORDER BY keyword followed by the column name, then ASC or DESC keywords specifying the order-ascending (A-Z) or descending (Z-A) respectively.

For example, SELECT column_name FROM table_name ORDER BY column_name ASC will display records based on the column_name in ascending order, A-Z.

Sorting data according to more than one column

In some cases, we may need to organize data based on the order of two or more columns. In this case, we can use the ORDER BY keyword followed by the first column name, then a comma, followed by the second column name and so on.

For instance, SELECT column1, column2, column3 FROM table_name ORDER BY column1, column2 DESC will display data sorted by column1, then by column2 in descending order. Column3 will remain unsorted.

Searching for values matching a certain pattern

Sometimes, instead of data that exactly matches a specific value, we might need to search for values related to a particular pattern. For instance, suppose we need to retrieve data with a particular word or phrase in its name.

In that case, we can use the LIKE keyword followed by a wildcard symbol (%) to represent the pattern. For example, SELECT column_name FROM table_name WHERE column_name LIKE ‘%pattern%’ will retrieve all rows containing the specified pattern.

Final Thoughts

Retrieving and filtering data from a database is crucial and allows businesses to make informed decisions. The keywords and techniques used for retrieving and filtering data from a database discussed above are fundamental.

Understanding how to use them can significantly enhance data analysis. The ORDER BY keyword is essential when it comes to sorting data, while the LIKE keyword is necessary when searching for data based on a specific pattern.

With this knowledge, we can find and organize data efficiently, leading to better business decisions.

Joining Data

Retrieving data from one table is useful, but when working with larger datasets, we may need to combine data from multiple tables. In this section, we will explore different ways of joining data.

Joining values from text columns into one string

Sometimes, we may require values from different columns to be combined into one string column. The CONCAT keyword concatenates two or more string columns.

We use the CONCAT keyword followed by the column names separated by commas, e.g., CONCAT(column1, ‘ ‘, column2) will combine column1 and column2 and put a space between them. The result will be a string with column1’s value followed by a space, then column2’s value.

Using mathematical operators

Mathematical operators such as +, -, *, / can be used to perform mathematical operations on values in the database. For instance, SELECT column1 + column2 FROM table_name will retrieve the sum of the values in column1 and column2.

Similarly, subtraction, multiplication, and division operations are performed by substituting the “+” sign with ” – “, “*”, and “/”.

Adding data from different tables

Suppose we have data stored in multiple tables, and we would like to combine them into one table. In that case, we can use the UNION and UNION ALL statements.

UNION returns only distinct values from tables, whereas UNION ALL returns all rows. For example, SELECT column1 FROM table1 UNION ALL SELECT column2 FROM table2 will retrieve data from column1 of table1, and column2 of table2 and combine them into one table.

Finding intersection of sets of data with INTERSECT, MINUS, and EXCEPT

Sometimes, we may want to find the overlapping values in different tables. We can do so using the INTERSECT keyword.

The INTERSECT keyword returns only the rows that exist in two or more queries. For instance, SELECT column1 FROM table1 INTERSECT SELECT column1 FROM table2 will retrieve only columns that exist in both table1 and table2.

The MINUS or EXCEPT keyword works similarly to the INTERSECT keyword. However, the MINUS keyword retrieves data present in the first table but absent in the second table.

For example, SELECT column_name FROM table1 MINUS SELECT column_name FROM table2 retrieves only data present in table1 but not in table2.

Joining data from different tables with aliases

Suppose we have columns with the same names in different tables we want to join, but we still need to identify the table from which a particular column came from. In that case, we can use aliases.

To use aliases, we can specify an alternative name for a table or column using the AS keyword. For example: SELECT t1.column1, t2.column2 FROM table1 AS t1, table2 AS t2 WHERE = will retrieve data from column1 and column2 from table1 and table2, respectively.

Aggregate Functions

Aggregate functions return a single value calculated from multiple rows of a table.

Counting the number of rows in a table

The COUNT function returns the count of the number of rows with a non-null value. For example, SELECT COUNT(*) FROM table_name returns the number of rows in the table_name.

Calculating the average of the values in a column

The AVG function calculates the average or mean of a column’s numeric values. For instance, SELECT AVG(column_name) FROM table_name.

Calculating the sum of the values in a column

The SUM function calculates the sum of a column’s numeric values. For example: SELECT SUM(column_name) FROM table_name.

Finding the minimum value in a column

The MIN function finds the smallest value in a column. For instance, SELECT MIN(column_name) FROM table_name.

Finding the maximum value in a column

The MAX function finds the largest value in a column. For example, SELECT MAX(column_name) FROM table_name.

Calculating the aggregate value for groups of records

We might want to calculate the aggregate values for specified groups within a table. For instance, the GROUP BY clause groups rows based on column values, and the aggregate functions operate on these groups.

For example: SELECT column_name, COUNT(*) FROM table_name GROUP BY column_name.

Filtering rows using aggregate functions

The HAVING keyword filters rows from a query based on the aggregated value of a column. For instance, SELECT column_name FROM table_name GROUP BY column_name HAVING COUNT(*) > 1; retrieves records with a count of columnname greater than 1.

Final Thoughts

In conclusion, joining data and using aggregate functions are essential when we need to process large data sets from databases. The CONCAT function is useful when combining text columns into one string.

We can use UNION, INTERSECT, MINUS, EXCEPT, and aliases to combine or retrieve data from different tables. Meanwhile, aggregate functions such as COUNT, AVG, SUM, MIN, MAX, GROUP BY, and HAVING are fundamental in processing large datasets.

By employing these techniques, data analysts can effectively manage and analyze data from multiple sources and get meaningful insights to drive informed decisions.

Modifying Data

Retrieving data from a database table is important, but its also necessary to have mechanisms to modify data. The following methods can be used to update and remove data after retrieving it.

Removing data from a table

Deleting data from tables requires the use of the DELETE FROM statement. The DELETE FROM statement removes data from the table entirely.

For example, DELETE FROM table1 will delete all data present in table1.

Removing records meeting a certain condition from a table

In some cases, we may only want to remove specific data records from a table. We can use the WHERE clause along with the DELETE statement.

The DELETE statement deletes only data from the table meeting the specified condition. For example, DELETE FROM table_name WHERE column_name=value will delete all rows where the column_name’s value equals value.

Inserting data into a table

Inserting new data into a table is critical when new information needs to be added. The INSERT INTO statement is used to insert data into a table.

For example: INSERT INTO table1 (column1, column2) VALUES (value1, value2). Column1, column2 represents columns in the table, and value1 and value2 are the respective values to be inserted.

Updating a column in a table

Sometimes, we may need to update a column’s value without deleting any data. This involves using the UPDATE statement.

For example: UPDATE table_name SET column_name=new value WHERE column_name=old value sets the column_name value to new value where the column_name value is old value.

Updating a column by filtering records

Updating data based on specific records involves combining a WHERE clause with the UPDATE statement to filter the records we need. We can update only some rows that match some condition.

For instance, UPDATE table_name SET column_name=new value WHERE column_name=old value.

Creating and Deleting Tables

Creating a table

To create a table, we use the CREATE TABLE statement, followed by the table name and the columns and their respective data types. For example, CREATE TABLE table1 (column1 type1, column2 type2, column3 type3).

Here, column1, column2, and column3 represent the columns in the table, and type1, type2, and type3 represent their respective data types such as integer, text, or date type.

Deleting a table

To remove a table entirely, we use the DROP TABLE statement followed by the table name. For example, DROP TABLE table1 deletes the table1 and all its contents.


In conclusion, modifying data in a database is essential when working with dynamic datasets. The DELETE statement helps remove either a specific record or all the data present in the table.

The INSERT INTO statement permits adding new data to a table, while the UPDATE statement lets us set new values for a column in a table. Ultimately, creating and deleting tables with the CREATE TABLE statement and DROP TABLE statement removes the need to manually produce or dismantle missing tables.

By using these statements, analysts can effectively work with data sets, maintaining the accuracy and consistency of the data. This article covers essential topics related to working with databases, including retrieving and filtering data, joining data, using aggregate functions, modifying data, and creating and deleting tables.

Retrieving and filtering data from a database are crucial for processing and analyzing large datasets. Joining data from different tables can streamline data processing, while using aggregate functions provides meaningful insights.

Modifying data ensures data accuracy, and creating and deleting tables helps maintain database stability. Overall, these techniques are essential in effectively managing and processing data, enabling informed decision-making in various fields.

By using these techniques, analysts can work more efficiently, consistently delivering accurate and reliable insights to their organizations.