Fix conversion failures in dates and typed parameters

Understand the likely causes

• Text is being interpreted as a typed value at a driver parameter, expression, comparison, assignment, or destination column. Implicit conversions follow SQL Server type-precedence rules and may occur somewhere other than the projection you first inspect.
• Unicode loss and conversion failure are related input-contract concerns but are not the same symptom. A missing N prefix or non-Unicode parameter can damage text before it reaches an nvarchar destination.

Trace types across the whole boundary

Record the source format, driver parameter type and length, stored procedure signature, intermediate variables, and target column type, precision, and scale. Use synthetic reproductions instead of pasting personal or financial input into a public issue. Do not assume a wider destination repairs text already truncated in an intermediate variable. Inspect implicit conversion warnings and expression types when the error seems unrelated to the displayed column.

Identify invalid text without changing rows

TRY_CONVERT returns NULL for a supported conversion that fails, allowing a read-only investigation of known-format text. It can still error for explicitly disallowed conversions. The example uses style 112 for an agreed eight-digit date and decimal parsing for a simple numeric contract. It distinguishes malformed input from valid typed values; a NULL source and an invalid source still need separate classification in real imports.

SELECT RawDate,TRY_CONVERT(date,RawDate,112) AS parsed_date,
 RawAmount,TRY_CONVERT(decimal(10,2),RawAmount) AS parsed_amount
FROM (VALUES (N'20260131',N'19.95'),
 (N'20260230',N'not-a-number'))v(RawDate,RawAmount);

RawDate  parsed_date RawAmount    parsed_amount
20260131 2026-01-31  19.95        19.95
20260230 NULL        not-a-number NULL

Repair the input contract at its source

Prefer a driver date parameter for a date and an explicitly sized decimal parameter for an amount. If importing text, agree an invariant format and an explicit conversion style, then quarantine rejected rows with reasons. Do not change the whole server's language to accommodate one file. Do not coalesce failed conversions to zero or a sentinel date unless that transformation is an explicit business policy.

Preserve Unicode through every stage

Use nvarchar parameters and columns for the course's multilingual contract and N-prefixed Unicode literals in T-SQL. Length n measures UTF-16 byte-pairs, so supplementary characters need careful boundary testing. SQL Server 2019 UTF-8 varchar is an explicit collation alternative, not a property of every varchar. Bind values rather than concatenating them into dynamic SQL; parameterization also prevents apostrophes from becoming command syntax.

Keep it from recurring

• Version type signatures with the API and schema. Keep import formats, timezone conventions, numeric scales, and Unicode lengths documented. Test culture differences at the boundary, and retain failed-row evidence without exposing sensitive original data publicly.