# SQL syntax — what works, what doesn't

The `INSQL` OLE DB provider supports a subset of T-SQL. Knowing the cliffs in advance saves a lot of "mysteriously empty rowset" debugging.

## Quick capability matrix

| Syntax element | Four-part / view | `OPENQUERY` |
| --- | --- | --- |
| `ORDER BY` | ✅ | ❌ inside the body. Wrap and order outside. |
| `GROUP BY` | ✅ | ❌ |
| `TagName IN (...)` | ✅ | ✅ |
| `TagName LIKE '...'` | ✅ | ✅ |
| Date/time functions (`DateAdd`, `GETDATE`, …) | ✅ | ✅ |
| `MIN`, `MAX`, `AVG`, `SUM`, `STDEV` | All | `MIN`, `MAX`, `AVG`, `SUM` only — no `STDEV` |
| Sub-`SELECT` (one normal SQL Server table + one extension table) | ✅ with restrictions | ❌ |
| Sub-`SELECT` (two extension tables) | ❌ | ❌ |
| Variables (`@foo`) | ✅ outside `OPENQUERY` | ❌ inside |

## Time-domain extensions (the `wwXxx` columns)

The Historian exposes "virtual columns" on every extension table that double as input parameters. They control retrieval semantics:

```text
wwCycleCount       wwEdgeDetection    wwFilter           wwInterpolationType
wwOption           wwQualityRule      wwResolution       wwRetrievalMode
wwStateCalc        wwTimeDeadband     wwTimeStampRule    wwTimeZone
wwValueDeadband    wwValueSelector    wwVersion
```

Reserved / deprecated: `wwParameters`, `wwMaxStates` (reserved), `wwRowCount` (deprecated — use `wwCycleCount`).

Set them in the `WHERE`:

```sql
SELECT DateTime, Value
FROM History
WHERE TagName = 'SysTimeSec'
  AND DateTime >= '2001-12-02 10:00:00'
  AND DateTime <= '2001-12-02 10:02:00'
  AND wwResolution    = 10
  AND wwRetrievalMode = 'cyclic';
```

**Three rules that bite:**

1. The provider is stateless. **Every extension parameter must be set in every query** that needs it — there is no session memory.
2. **`IN` and `OR` cannot multiplex an extension.** `wwVersion IN ('original','latest')` and `wwRetrievalMode = 'Delta' OR wwVersion = 'latest'` both fail.
3. Values are **case-insensitive** even on a case-sensitive SQL Server collation.

Each individual extension is documented in [`04-retrieval-options.md`](04-retrieval-options.md).

## `LIKE` — only `TagName` and `Value`

`LIKE` only works on the `TagName` and `Value` columns (and `Value LIKE` only against a string tag).

```sql
SELECT TagName, Value
FROM History
WHERE TagName LIKE 'Sys%'
  AND DateTime > '1999-05-24 14:30:00'
  AND DateTime < '1999-05-24 14:32:00';
```

## `IN` against `AnalogTag` / `DiscreteTag` / `StringTag` with `LIKE`

If you push a `LIKE` *inside* an `IN`-driven sub-`SELECT` against the typed tag tables, the query fails **unless you select the `vValue` column**. An `INNER REMOTE JOIN` is more efficient anyway:

```sql
-- Clunky (and only works if vValue is in the projection):
SELECT DateTime, TagName, vValue, Quality, QualityDetail
FROM History
WHERE TagName IN (SELECT TagName FROM StringTag WHERE TagName LIKE 'SysString')
  AND DateTime BETWEEN '2001-06-21 16:00:00.000' AND '2001-06-21 16:40:00.000'
  AND wwRetrievalMode = 'Delta';

-- Preferred:
SELECT h.TagName, DateTime, Value
FROM SnapshotTag st
INNER REMOTE JOIN INSQL.Runtime.dbo.History h
  ON st.TagName = h.TagName
WHERE EventTagName = 'SysStatusEvent'
  AND DateTime = '2001-12-20 0:00';
```

## Joins

- **Inside `OPENQUERY`: not supported.** Both implicit (`FROM Tag t, Live v`) and explicit (`JOIN`) joins fail. Always pull the join out:

  ```sql
  SELECT v.DateTime, v.TagName, v.Value, e.Unit
  FROM OPENQUERY(INSQL, '
    SELECT DateTime, TagName, Value FROM Live
    WHERE TagName LIKE "%Date%"
  ') v
  JOIN AnalogTag t      ON v.TagName = t.TagName
  JOIN EngineeringUnit e ON t.EUKey   = e.EUKey
  ORDER BY t.TagName;
  ```

- **Joining a SQL Server table against an extension table: use `INNER REMOTE JOIN`** with the SQL Server table on the left and the extension table on the right:

  ```text
  <SQLServerTable> INNER REMOTE JOIN <HistorianExtensionTable>
  ```

  This forces SQL Server to send the join predicate down to the OLE DB provider in one round trip rather than dragging the whole extension table back to the engine.

## `CONVERT` on `vValue` inside `OPENQUERY`

`CONVERT(...)` against the variant `vValue` column is **not supported inside the `OPENQUERY` body**. Filter outside:

```sql
SELECT * FROM OPENQUERY(INSQL, '
  SELECT DateTime, Quality, OPCQuality, QualityDetail, Value, vValue, TagName
  FROM History
  WHERE TagName IN ("SysTimeMin", "SysPulse")
    AND DateTime >= "2001-12-30 04:00:00.000"
    AND DateTime <= "2001-12-30 09:00:00.000"
    AND wwRetrievalMode = "Delta"
') 
WHERE CONVERT(float, vValue) = 20.0;
```

## Sub-SELECTs against a SQL Server table + extension table

Permitted but inefficient — SQL Server fans the whole sub-query out badly. Always rewrite as `INNER REMOTE JOIN`. Two-extension-table sub-SELECTs are not supported at all.

## Optimizer dropping criteria

The SQL Server optimizer sometimes decides a `WHERE` clause is "redundant" and never sends it to `INSQL`, producing wrong-but-not-erroring results. Diagnose with the SSMS query plan; remediate by rewriting so the criteria flow through a more direct path, e.g. **put the small driver table on the left of `INNER REMOTE JOIN`** so its tagnames flow into the right side:

```sql
DECLARE @TagList TABLE (TagName nvarchar(256));
INSERT @TagList SELECT 'SysTimeSec' UNION SELECT 'SysPerfCPUTotal';

-- Wrong: TagName criteria gets optimized out
SELECT DateTime, h.vValue, h.TagName
FROM History h
INNER REMOTE JOIN @TagList l ON h.TagName = l.TagName
WHERE DateTime >= DATEADD(hour, -1, GETDATE()) AND DateTime < GETDATE()
  AND wwRetrievalMode = 'AVG' AND wwCycleCount = 1;

-- Right: driver table on the left, criteria reach the OLE DB provider
SELECT DateTime, h.vValue, h.TagName
FROM @TagList l
INNER REMOTE JOIN History h ON h.TagName = l.TagName
WHERE DateTime >= DATEADD(hour, -1, GETDATE()) AND DateTime < GETDATE()
  AND wwRetrievalMode = 'AVG' AND wwCycleCount = 1;
```

## Variant columns + functions

If you wrap a variant column (`vValue`) with a function like `ROUND()`, SQL Server emits a syntax error that **doesn't propagate back through OLE DB** — you get silently wrong / empty rows. Cast first or filter on the typed `Value` column.

## `StartDateTime` cannot drive query criteria

Many retrieval modes return a `StartDateTime` for the cycle. You can `SELECT` it, but you **cannot use it in `WHERE`** — the predicate is silently ignored. Use `DateTime` only.

## Comparisons and `NULL`

A predicate like `Value > 0` will error when the provider returns a `NULL`. Always pair it with `AND Value IS NOT NULL`.

## `WHERE` clause anomalies

If the SQL Server optimizer decides your `WHERE` is "useless" (typically a `LIKE '%'` against `TagName`), it removes the clause entirely and you get the OLE DB error from [`01-overview.md`](01-overview.md). Add another harmless predicate to keep the clause "needed":

```sql
SELECT DateTime, Value, QualityDetail
FROM History
WHERE TagName LIKE '%'
  AND wwRetrievalMode = 'delta';   -- forces the WHERE to survive optimization
```

## Compatibility notes for the older `datetime` type

If your client uses the older SQL Server `datetime` type (instead of `datetime2`), expect rounding to ~3.3 ms vs. the 100 ns native `FILETIME` resolution.

## What `OPENQUERY` and Microsoft Query do *not* talk

Microsoft Query (the legacy Excel/Access tool) cannot parse `OPENQUERY` statements. Use one of the other connection paths from Excel.

## Next

- [`03-retrieval-modes.md`](03-retrieval-modes.md) — every `wwRetrievalMode` value.
- [`04-retrieval-options.md`](04-retrieval-options.md) — every other `wwXxx` parameter.