row and column constraints 行和列的约束
In other words, schemas are not the equivalent of SQL column constraints.
换句话说，模式并不等同于sql列约束。
Constraints come in two types: table-level constraints and column constraints.
约束有两种类型：表级约束和列约束。
This new validation will enforce NOT null column constraints by rejecting null values, but only in situations where your database would reject a null value.
这种校验会通过拒绝空值，强迫加上非空[not NULL]字段的约束，但是也只有在这种情况下，你的数据库会拒绝空值。
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