Document differences in CAST(... AS BIGINT)

[AnyhowStep]

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SELECT 
  CAST(1.1 AS BIGINT),
  CAST(1.5 AS BIGINT),
  CAST(1.7 AS BIGINT),
  CAST(-1.1 AS BIGINT),
  CAST(-1.5 AS BIGINT),
  CAST(-1.7 AS BIGINT);

---

PostgreSQL,

int8	int8	int8	int8	int8	int8
1	2	2	-1	-2	-2

---

SQLite,

CAST(1.1 AS BIGINT)	CAST(1.5 AS BIGINT)	CAST(1.7 AS BIGINT)	CAST(-1.1 AS BIGINT)	CAST(-1.5 AS BIGINT)	CAST(-1.7 AS BIGINT)
1	1	1	-1	-1	-1

---

MySQL,

CAST(1.1 AS SIGNED)	CAST(1.5 AS SIGNED)	CAST(1.7 AS SIGNED)	CAST(-1.1 AS SIGNED)	CAST(-1.5 AS SIGNED)	CAST(-1.7 AS SIGNED)
1	2	2	-1	-2	-2

[AnyhowStep]

SQLite truncates.

MySQL and PostgreSQL round.