How can I show each group’s total beside every detail row?

Use a windowed SUM, partitioned by the customer column. In SQLite, aggregate functions can be used as window functions by adding an OVER clause; a partitioned window sum computes a group total while retaining the result rows. [S1]

Assume a table named invoice_lines with customer, invoice_no, and line_amount columns. This query keeps each line and adds its customer’s total:

SELECT
  customer,
  invoice_no,
  line_amount,
  SUM(line_amount) OVER (
    PARTITION BY customer
  ) AS customer_total
FROM invoice_lines
ORDER BY customer, invoice_no;

PARTITION BY customer defines which rows are included in each total. The window calculation does not combine those rows into one row per customer. [S1]

Here is an invented example, assuming the table contains these three invoice lines:

customer  invoice_no  line_amount
Alder     101         12.00
Birch     202          7.50
Alder     102          8.00

The query returns:

customer  invoice_no  line_amount  customer_total
Alder     101         12.00        20.00
Alder     102          8.00        20.00
Birch     202          7.50         7.50

Both Alder detail rows remain visible, and each displays the same Alder total. The Birch row displays Birch’s total. The example assumes those are the complete rows being queried and uses ordinary numeric amounts; adjust the names and columns to match your table.

GROUP BY customer is useful when you want one summarized row per customer, but it does not by itself preserve each invoice line in that result. The window version gives you both the detail columns and a repeated customer total. [S1]

Use the window sum when you need to inspect or export detail rows alongside totals. Use a grouped aggregate when the desired output is only summaries. The relevant distinction is whether you want the detail rows retained or a grouped result. [S1]

Start with this template, replacing the bracketed placeholders with your actual table and column names:

SELECT
  [group_column],
  [detail_column],
  [amount_column],
  SUM([amount_column]) OVER (
    PARTITION BY [group_column]
  ) AS group_total
FROM [table_name];

Keep the original amount in the result if readers need to compare each detail value with the repeated total. Add an ORDER BY when you want a particular display order; it does not replace the PARTITION BY that defines the total.

Before relying on the result, check that your chosen partition column expresses the intended group and that the query is run against the rows you mean to total. The example does not establish how your application stores amounts or handles missing values, so inspect your data and requirements if those details matter.