SQL : PostgreSQL

  Aggregate Functions Like most other relational database products,  PostgreSQL  supports  aggregate functions . An aggregate function computes a single result from multiple input rows. For example, there are aggregates to compute the  count ,  sum ,  avg  (average),  max  (maximum) and  min  (minimum) over a set of rows. As an example, we can find the highest low-temperature reading anywhere with: SELECT max(temp_lo) FROM weather; max ----- 46 (1 row) If we wanted to know what city (or cities) that reading occurred in, we might try: SELECT city FROM weather WHERE temp_lo = max(temp_lo); WRONG but this will not work since the aggregate  max  cannot be used in the  WHERE  clause. (This restriction exists because the  WHERE  clause determines which rows will be included in the aggregate calculation; so obviously it has to be evaluated before aggregate functions are computed.) However, as is o...
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Databases in data analytics

 


Databases in data analytics

Databases enable analysts to manipulate, store, and process data. This helps them search through data a lot more efficiently to get the best insights. 


Relational databases

A relational database is a database that contains a series of tables that can be connected to show relationships. Basically, they allow data analysts to organize and link data based on what the data has in common. 

In a non-relational table, you will find all of the possible variables you might be interested in analyzing all grouped together. This can make it really hard to sort through. This is one reason why relational databases are so common in data analysis: they simplify a lot of analysis processes and make data easier to find and use across an entire database. 

The key to relational databases

Tables in a relational database are connected by the fields they have in common. You might remember learning about primary and foreign keys before. As a quick refresher, a primary key is an identifier that references a column in which each value is unique. In other words, it's a column of a table that is used to uniquely identify each record within that table. The value assigned to the primary key in a particular row must be unique within the entire table. For example, if customer_id is the primary key for the customer table, no two customers will ever have the same customer_id. 

By contrast, a foreign key is a field within a table that is a primary key in another table. A table can have only one primary key, but it can have multiple foreign keys. These keys are what create the relationships between tables in a relational database, which helps organize and connect data across multiple tables in the database.

Some tables don't require a primary key. For example, a revenue table can have multiple foreign keys and not have a primary key. A primary key may also be constructed using multiple columns of a table. This type of primary key is called a composite key. For example, if customer_id and location_id are two columns of a composite key for a customer table, the values assigned to those fields in any given row must be unique within the entire table.

Image of 5 tables: customer table, revenue table, branch table, date table, and product table

SQL? You’re speaking my language 

Databases use a special language to communicate called a query language. Structured Query Language (SQL) is a type of query language that lets data analysts communicate with a database. So, a data analyst will use SQL to create a query to view the specific data that they want from within the larger set. In a relational database, data analysts can write queries to get data from the related tables. SQL is a powerful tool for working with databases — which is why you are going to learn more about it coming up!


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SQL : PostgreSQL

  Aggregate Functions Like most other relational database products,  PostgreSQL  supports  aggregate functions . An aggregate function computes a single result from multiple input rows. For example, there are aggregates to compute the  count ,  sum ,  avg  (average),  max  (maximum) and  min  (minimum) over a set of rows. As an example, we can find the highest low-temperature reading anywhere with: SELECT max(temp_lo) FROM weather; max ----- 46 (1 row) If we wanted to know what city (or cities) that reading occurred in, we might try: SELECT city FROM weather WHERE temp_lo = max(temp_lo); WRONG but this will not work since the aggregate  max  cannot be used in the  WHERE  clause. (This restriction exists because the  WHERE  clause determines which rows will be included in the aggregate calculation; so obviously it has to be evaluated before aggregate functions are computed.) However, as is o...
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