MySQL 5.7NULLand''Multidimensional analysis of empty character values
introduction
Correctly understand and use in database design and developmentNULLValues are essential to ensure data quality and query efficiency.NULLValues represent unknown or missing data states, and have a unique way of processing in MySQL 5.7. This article will pair from multiple dimensionsNULLValues are analyzed in depth and with empty strings''and other controls are compared to provide readers with a comprehensive and clear understanding.
1. Storage dimension
NULLvalue:
- Each allowed
NULLThe columns allocate additional space to mark whether the field isNULL. Specifically, MySQL uses bitmaps to track which fields areNULLof. - For the InnoDB storage engine,
NULLValues may occupy a zero-byte portion of the data, but a fixed number of bytes is reserved in fixed-length columns. This meansNULLThe actual storage cost of a value depends on the storage engine and column type.
Empty string''：
- An empty string is a specific value and does not require additional space to identify its state. It only takes up the minimum space required to represent the string.
- for
CHARColumns of type, even if an empty string is inserted, MySQL will be filled according to the length defined by the column and remove trailing spaces when searching; and forVARCHARType, it will be saved directly according to the actual input content and will not be filled.
2. Query performance dimensions
NULLvalue:
- Query
NULLSpecial operators must be used when values areIS NULLorIS NOT NULL,becauseNULLIt represents logical uncertainty. - This query may be slightly slower than normal equivalents, especially when large amounts of data are involved, as the database needs to check additional bitmap information to determine which fields are
NULLof.
Empty string''：
- Empty strings can be directly compared like any other string, which means you can use standard relational operators (e.g.
=、<>etc.) to find or filter out records with empty string values. - Such queries are usually faster because no additional processing steps are required, but frequent queries on empty strings on index columns may affect index efficiency.
3. Index efficiency dimension
NULLvalue:
- Although MySQL supports it in the
NULLcreate indexes on columns, but this does not meanNULLValues can utilize indexes as efficiently as normal values. -
NULLThe presence of values may cause index selectivity to decrease, which in turn affects query performance. Some types of indexes (such as unique indexes) do not allow duplicates.NULLvalue, which further limitsNULLvalue application scenario.
Empty string''：
- As a specific value, empty strings can be well integrated into the index structure, avoiding
NULLThe complexity brought by values, in theory, such a design may improve the efficiency of the index. - If a column allows
NULLBut you chose to use an empty string insteadNULL, then this method can help simplify query logic and reduce unnecessary conditional judgments.
IV. Aggregation function processing dimensions
NULLvalue:
- Most aggregate functions (e.g.
COUNT()、SUM()、AVG()Will ignore itNULLvalue. This is important to ensure statistics accuracy, but it also means you need to think extra about how to deal with those that may containNULLcolumn of value. - For example, when calculating the average, if the value of some rows is
NULL, directly calculating the average value may lead to inaccurate results.
Empty string''：
- An empty string is treated as a valid value and counted as the result of the aggregate function.
- For example,
COUNT(*)counts the number of all rows, andCOUNT(column)It will be excludedNULLBut include empty strings. When writing aggregate queries, it is important to know if there are a large number of empty strings in your dataset, as they may have an impact on the results you end up with.
5. Business logic meaning dimension
NULLvalue:
- From a business perspective,
NULLValues are usually used to represent missing or unknown states of data. For example, in the user information table, if a user does not provide a mobile phone number, the value of the field can be set toNULL。 -
NULLValues can also be used to express "not applicable" situations where a certain attribute does not exist at all for a particular object. This approach not only helps keep data consistent and complete, but also helps developers to make it easier to distinguish which users do provide information and which do not.
Empty string''：
- Empty strings are more suitable for representing situations where there is but the content is empty. For example, in the note description field, even if the user does not fill in the specific content, the field can be set to an empty string to indicate that the field has been considered but has been left blank.
- It should be noted, however, that over-reliance on empty strings can complicate the data model, especially if it requires frequent checks on whether it is empty.
For clearer comparisonNULLand empty strings''In MySQL 5.7, we can summarize the above analysis into a table. The following is a comparison based on five dimensions: storage, query performance, index efficiency, aggregation function processing, and business logic significance:
| Dimension | NULL value | Empty string'' |
|---|---|---|
| storage | -Each per allowedNULLThe columns will allocate additional space to mark whether or notNULL- For InnoDB, NULLMay occupy the zero-byte data part, but reserve a fixed number of bytes in the fixed length column |
- No additional space is required to identify the status - Compact storage, taking up only the minimum space required to represent a string |
| Query performance | - Need to useIS NULLorIS NOT NULLOperator- Query may be slightly slower than normal equivalents |
- Can be compared directly like other strings - Query is usually faster |
| Index efficiency | - May cause index selectivity to decrease - The unique index does not allow duplication NULLvalue |
- Better integrate into the index structure - Does not cause index complexity |
| Aggregation function | - Most aggregate functions are ignoredNULLvalue- COUNT()Statistics onlyNULLentry |
- considered valid values and counted into the results - COUNT(column)Include empty strings |
| Business logic | - Indicates that data is missing or unknown - Applicable to "not applicable" situations |
- indicates that there is but the content is empty - Suitable for note description field |
Summarize
Through the analysis of the above five dimensions, we can seeNULLand empty strings''There is a clear difference in MySQL 5.7. These differences are reflected in storage, query performance, index efficiency, aggregation function processing, and business logic. Understanding these differences helps us make smarter choices when designing database structures, write more efficient SQL queries, and ensure quality and consistency of data.
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