Query

How you read data in Osy#. You write C# LINQ; it becomes one SQL statement. The rules that follow from that are the whole model: the predicate runs…

A parent's child collection — `order.Lines` — is not a loaded array. It is a QUERY, correlated to that parent, and every LINQ verb works on it:…

Ends a query chain with a set-based DELETE: every row the chain selects is deleted in the database, immediately, in one statement — and the call…

Removes duplicate rows from a query result. On whole entity rows it is a no-op, because rows are already unique by Id — it earns its keep on…

Filter a query by membership in a RUNTIME list — list.Contains(e.Column) inside a Where lowers to SQL `= ANY(@param)`, passing the whole list as one…

The terminals that return ONE row. They differ in what they promise, and choosing the wrong one is how a bug hides: `Single` asserts there is exactly…

Group rows by a key and reduce each group to one row — `g.Key`, `g.Count()`, `g.Sum/Average/Min/Max(…)` — computed by the database as a real GROUP…

Pre-load the related rows a query's results are about to navigate to. `Include(o => o.Lines)` does not change what comes back — the same rows, the…

Ends a query chain by creating one row of ANOTHER entity per row the chain selects — an INSERT … SELECT in one statement. The chain is the source…

Combine two entities into one result. `Join` keeps the rows that match on both sides; `LeftJoin` keeps every row on the left and gives you null on…

Query a local `List<T>`, `HashSet<T>` or `T[]` — of your own `class` values OR of plain scalars like `string[]` and `int[]` — with the same LINQ…

Sort a query by one key or several. `OrderBy`/`OrderByDescending` start the sort, `ThenBy`/`ThenByDescending` add further keys — and the keys…

Holds a query instead of its rows. A clause you write against a `Query<T>` joins the query rather than filtering rows already fetched, so a chain…

Reshape what a query returns: one column, an anonymous row, or a `class` you declared. The projection becomes the SQL SELECT list, so the columns you…

Page a query with Skip(n) (OFFSET) and Take(m) (LIMIT). The count can be a compile-time constant OR a runtime integer — a variable, parameter, or…

A sequence the client already holds — a component's `T[]` rows parameter, a `List<T>` — sorts with `OrderBy` / `OrderByDescending`, and the key is…

Fold rows down to a single number — a query or a `List<T>` you already hold. The one thing to know before you use them: **Min/Max/Average answer null…

Runs the query and materialises the rows as a `List<Entity>`. Until you call it, a query is a description of what you want; ToList is the moment it…

Walk a relation recursively — an org chart up to its root, a category tree down to its leaves, a bill of materials, a reply thread — and get back…

Combines two row-queries over the same entity with SQL set semantics: Union dedups, Concat keeps duplicates (UNION ALL), Intersect keeps rows present…

Ends a query chain with a set-based UPDATE: every row the chain selects gets the assignments applied, in the database, immediately — and the call…

Query an entity by writing a predicate over it. The query runs in the database — not a filter over rows you already fetched — so a table with…

Ranking and neighbours inside a query's result. The indexed `Select((s, i) => …)` over an ordered chain is C#'s own spelling of a row number; the…