Wren SQL

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Wren Engine translates SQL through a semantic layer (MDL — Model Definition Language) before executing it against a backend database. SQL must target MDL model names, not raw database tables.

For specific topics, load the relevant reference file:

Topic	Reference
SQL error diagnosis and correction	references/correction.md
Date/time functions and intervals	references/datetime.md
ARRAY, STRUCT, JSON/VARIANT types	references/types.md
BigQuery dialect quirks	references/bigquery.md

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Context

• You are querying a semantic layer, not a database directly.
• Only use model/view/column names defined in the MDL — never raw database table references.
• Wren Engine uses a generic SQL dialect similar to ANSI SQL (DataFusion/Postgres/DuckDB), but with differences.
• Check the dataSource field to identify the backend and apply dialect-specific rules if needed.

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Core SQL Rules

• Only SELECT statements. No DELETE, UPDATE, INSERT.
• Only use tables and columns from the MDL schema.
• Do not include comments in generated SQL.
• Prefer CTEs over subqueries.
• Identifiers are case-sensitive. Quote identifiers containing unicode, special characters (except _), or starting with a digit using double quotes.
  • Examples: "客户"."姓名", "table-name"."col", "123column"
• Identifier quotes: " (double quotes). String literal quotes: ' (single quotes).
• For specific date queries, use a range:

  WHERE ts >= CAST('2024-11-01 00:00:00' AS TIMESTAMP WITH TIME ZONE)
    AND ts <  CAST('2024-11-02 00:00:00' AS TIMESTAMP WITH TIME ZONE)
  

• For ranking, use DENSE_RANK() + WHERE. Include the ranking column in SELECT.
• Avoid correlated subqueries — use JOINs instead.
• Use SAFE_CAST when casting might fail: SAFE_CAST(col AS INT)

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Filter Strategies

Column type	Strategy
Text	LIKE '%value%' for partial match
Numeric	BETWEEN 30 AND 40
Date/Timestamp	>= '2024-01-01' AND < '2024-02-01'
Exact value	= or IN (...)
Primary key / indexed	Prefer equality (=)

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Supported Cast Types

bool, boolean, int, integer, bigint, smallint, tinyint, float, double, real, decimal, numeric, varchar, char, string, text, date, time, timestamp, timestamp with time zone, bytea

Example: CAST(col AS INT), TIMESTAMP '2024-11-09 00:00:00'

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Aggregation

• All non-aggregated SELECT columns must appear in GROUP BY (window functions excepted).
• Aggregate conditions go in HAVING, not WHERE.
• Prefer ordinal GROUP BY for long column names:

  SELECT very_long_column_name AS alias, COUNT(*) FROM t GROUP BY 1
  

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Sorting and Limiting

• ORDER BY for sort; LIMIT to restrict rows.
• When ORDER BY appears in a subquery or CTE, always include LIMIT.

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Subquery Patterns

• Prefer CTEs (WITH clause) over nested subqueries.
• Subquery in SELECT must return a single value per row.
• Subquery in WHERE: use IN, EXISTS, or comparison operators.
• IN SUBQUERY in JOIN conditions is not supported — use JOIN ... ON instead.
• RECURSIVE CTEs are not supported.

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Calculated Fields

Columns marked as Calculated Field in the MDL have pre-defined computation logic. Use them directly instead of re-implementing the calculation.

Read the column comment (e.g., column expression: avg(reviews.Score)) to understand what the field represents.

-- Schema has: Rating DOUBLE (Calculated Field: avg(reviews.Score))
--             ReviewCount BIGINT (Calculated Field: count(reviews.Id))

-- Correct — use Calculated Fields directly:
SELECT AVG(Rating) FROM orders WHERE ReviewCount > 10

-- Incorrect — do not re-join and re-aggregate manually