Select Data with Count in Postgresql

postgresql count rows with value
postgresql count multiple columns
postgresql count joined rows
postgres count group by
postgresql number of rows returned
postgres count rows with same value
postgresql count columns in database
postgres count number of occurrences

I want to ask how do I select data together with count data.

In this case, I want the user to appear and the number of transactions that he has.

Like this code that I made.

SELECT "transaction"."user_id",
COUNT(transaction.id) trans_count
FROM transaction 
inner join "users" on "users"."id" = "transaction"."user_id"
GROUP BY user_id

The code above successfully selects user_id and trans_count, but when I am trying to show users.name

this error message appears.

Error in query: ERROR: column "users.name" must appear in the GROUP BY clause or be used in an aggregate function LINE 3: "users"."name"

Is it true that I am cannot select other data when I count data or is there a better way ?.

Thank You.

You can include user.name in the group by:

SELECT "transaction"."user_id",
"user"."name",
COUNT(transaction.id) trans_count
FROM transaction 
inner join "users" on "users"."id" = "transaction"."user_id"
GROUP BY "transaction"."user_id", "user"."name"

Otherwise, when the DBMS tries to combine (group) multiple rows into a single row, it doesn't know which name value should it pick, which is why it throws the error.

In this case, user_id and user.name have a one-to-one mapping, so you can simply include name in the group by clause.

Otherwise you'd have to tell the DBMS how to select one value from the multiple records that are in each group, eg:

min(user.name) or max(user.name)

SELECT "transaction"."user_id",
min("user"."name") user_name,
COUNT(transaction.id) trans_count
FROM transaction 
inner join "users" on "users"."id" = "transaction"."user_id"
GROUP BY "transaction"."user_id"

Count estimate, PostgreSQL COUNT SELECT SELECT COUNT ( [*], [DISTINCT] [column_name] ) FROM TABLE_NAME; Let's dig a little deeper into the syntax shown above: SELECT – This is used to select certain columns from the database. COUNT – This is used to count the number of records in this table. PostgreSQL COUNT on specific column . If we want to get the number of employees who earns commission in the employee table, the following SQL can be used. SQL. Code: SELECT COUNT(commission) FROM employee; Output: Explanation. The above example shows that only 3 employees earn commission because the COUNT function ignored the NULL values. PostgreSQL COUNT DISTINCT

When every your using GROUP BY You must be group by all column which is fetching or use Aggregate Functions

Group by (same as @rohitvats)

GROUP BY "transaction"."user_id", "user"."name"

---- OR ----

Aggregate Functions MAX(), MIN()

SELECT "transaction"."user_id",
MAX("user"."name") as name,
COUNT(transaction.id) trans_count
FROM transaction 
inner join "users" on "users"."id" = "transaction"."user_id"
GROUP BY "transaction"."user_id"

How to Use the Postgres Count Select, The PostgreSQL COUNT function counts number of rows or non-NULL values SELECT COUNT ( DISTINCT designame) FROM employee;. SELECT no, name, price, "date" FROM (SELECT no, name, price, "date", COUNT(*) OVER (PARTITION BY name) AS cnt FROM product_price_info) AS t WHERE t.cnt > 1 You can use the window version of COUNT to get the population of each name partition. Then, in an outer query, filter out name partitions having a population that is less than 2.

Your code would work if you aggregated by the users.id:

SELECT u.id, u.user_name, COUNT(*) as trans_count
FROM users u JOIN
     transaction t
     ON t.id = u.user_id
GROUP BY u.id;

(I removed the double quotes because they clutter the logic and are not necessary to explain what is going on.)

Why? Presumably, users.id is unique (or equivalently the primary key). Postgres supports aggregating by a unique key in a table and also including unaggregated columns in the SELECT. This is an implementation of "functional dependent" aggregations from the SQL standard.

When you use transactions.user_id, Postgres does not recognize the functional dependence (even though you might think that the ON clause would imply it). So, your code doesn't work.

The alternative is to add user_name to the GROUP BY as well. However, your version almost works if you use the column from the correct table.

PostgreSQL COUNT function, Performance for an exact count grows linearly with table size. EXPLAIN SELECT count(*) FROM items;� Need to find out how many rows, or how much data your PostgreSQL tables are consuming? Possibly, looking to find out what the largest tables in the database are? There are a couple of options available depending on your needs. Single Table. Finding out how many rows are in a single table is the easiest task. Simply run the query:

Faster PostgreSQL Counting, I can use SELECT count(*) FROM table . But if my constant value is 500,000 and I have 5,000,000,000 rows in my table, counting all rows will� If you want to select data from all the columns of the table, you can use an asterisk (*) shorthand instead of specifying all the column names. The select list may also contain expressions or literal values. Second, specify the name of the table from which you want to query data after the FROMkeyword. The FROMclause is optional.

Fast way to discover the row count of a table in PostgreSQL, This article explores your options to make counting rows faster using data modifications have to wait until the SELECT count(*) is done. Date Thu 28 February 2019 By Rob Osterburg Tags postgres / postgresql / database / relational / notes / sql / command line / psql / pgadmin / aggregation / filtering / group / join / joining / joining tables / macOS / OS X / mac / apple / installation / import data / importing / sort / sorting / order / ordering / sub-queries / sub-query

PostgreSQL count(*) made fast, This PostgreSQL tutorial explains how to use the PostgreSQL count function with SELECT count(aggregate_expression) FROM tables [WHERE conditions];. SELECT year, COUNT(id) FROM YOUR_TABLE GROUP BY year HAVING COUNT(id) > 1 ORDER BY COUNT(id); Using the sql statement above you get a table which contains all the duplicate years in your table. In order to delete all the duplicates except of the the latest duplicate entry you should use the above sql statement.

Comments
  • When several tables are involved, it's good programming practice to qualify all columns, like "transaction"."user_id".
  • You typically GROUP BY the same columns as you SELECT, except those who are arguments to set functions.
  • I don't think there is a need for group by to call others. Thank you very helpful
  • Thank you for the explanation now I am understand, this works now !. :)
  • This is very helpful, after this i will learn more about the function you mean.
  • So like that. Thank you for your explanation is very easy to understand. :)