The SQL COUNT() function is a handy tool for telling us how many rows would be returned in a query. We can pass a column name to the function or we can pass the asterisk (*) wildcard to indicate all columns.
If a column contains NULL values, we could get different results, depending on whether we use the column name or the asterisk (*).
The SQL UNION operator concatenates the results of two queries into a single result set. By default it returns distinct rows (i.e. it removes any redundant duplicate rows from the result set). But we can also use UNION ALL to return non-distinct rows (i.e. retain duplicates).
When it comes to NULL values, it’s pretty straight forward. SQL treats two NULL values as non distinct values. In other words, they’re duplicates.
Continue readingIf you’ve ever used the UNION clause in a SQL query, chances are you used UNION ALL or just UNION by itself. But some RDBMSs also accept a UNION DISTINCT option.
The UNION DISTINCT option is basically the equivalent of UNION by itself. That is, it removes redundant duplicate rows.
When writing SQL queries, we’ll often use a WHERE clause or HAVING clause to narrow the results down to just those rows that we’re interested in.
But sometimes we might want to reduce the number of rows returned without adding extra filtering criteria. Sometimes we might just want to see a handful of rows, without hundreds, thousands or even millions of rows being returned.
Continue readingThe SQL DISTINCT clause allows us to remove redundant duplicate rows from our query results. For example if two rows contain the same values, then only one of them is returned.
But how does DISTINCT handle NULL values?
Let’s find out.
Continue readingIn statistics, the mode is the most commonly occurring value in a data set. When using SQL databases, we can easily run a SQL query to find out the mode of a given column.
Below are several options we can use to calculate the mode in SQL.
Continue readingMany SQL databases have a window function called NTILE() function that divides a rowset or partition into a given number of groups (buckets). The function typically returns the bucket number of the current row within its partition.
Most of the major relational database management systems (RDBMSs) allow for a DISTINCT clause to be included in our SQL queries.
We use the DISTINCT keyword to return only unique rows. It eliminates duplicates from the results. If we have two or more rows with exactly the same data, we’ll only see one row in the results.
The LAG() function is a window function that’s available in many SQL databases. It returns the value of a given expression from the row that lags (precedes) the current row by a given number of rows within its partition.
In other words, the LAG() function returns a value from a previous row.
Many relational database management systems (RDBMSs) have a LEAD() function that allows us to retrieve a value from a following row.
The SQL LEAD() function returns the value of a given expression from the row that leads (follows) the current row by a given number of rows within its partition.