UseCOMMENTONCOLUMNorALTERTABLEwithCOMMENTtodocumenttablesandcolumnsinSQL;syntaxvariesbyDBMS—PostgreSQLandOracleuseCOMMENTON,MySQLusesCOMMENTinCREATE/ALTERstatements,andcommentscanbeviewedviasystemtableslikeINFORMATION_SCHEMA,butSQLitelackssupport.

The SOUNDEX function converts text into a four-character code representing pronunciation, adds three digits to the first letter, ignores vowels and specific letters, and maps consonants with similar pronunciations to the same number, realizing pronunciation-based search. For example, Smith and Smythe both generate S530, and names with similar pronunciations can be found through WHERESOUNDEX(last_name)=SOUNDEX('Smith'). Combined with the DIFFERENCE function, it can return a similarity score of 0 to 4, filter the results of pronunciation close, which is suitable for dealing with spelling differences, but has limited effect on non-English names, and performance optimization needs to be paid attention to.

When using CREATETABLE, add UNIQUE keyword or use ALTERTABLEADDCONSTRAINT to add constraints to existing tables to ensure that the values ??in the column are unique, and support single columns or multiple columns. Before adding, you need to ensure that the data is not duplicated. You can delete it through DROPCONSTRAINT, pay attention to the syntax differences between different databases and NULL values.

Tofindorphanedrecords,useaLEFTJOINorNOTEXISTStoidentifychildrecordswithoutmatchingparentrecords.Forexample,SELECTo.FROMOrdersoLEFTJOINCustomerscONo.customer_id=c.customer_idWHEREc.customer_idISNULLreturnsorderslinkedtonon-existentcustomers.Alternativ

Use the LAST_DAY() function (MySQL, Oracle) to directly obtain the last day of the month where the specified date is, such as LAST_DAY('2023-10-15') to return 2023-10-31; 2. SQLServer uses the EOMONTH() function to achieve the same function; 3. PostgreSQL calculates the end of the month through DATE_TRUNC and INTERVAL; 4. SQLite uses the date function to combine 'startofmonth', '1month' and '-1day' to obtain the results.

SQLServerusesNEWID()togenerateGUIDs;2.MySQLusesUUID()forversion1UUIDs;3.PostgreSQLusesgen_random_uuid()afterenablingpgcrypto;4.SQLitelacksbuilt-insupport,sogenerateUUIDsinapplicationcode.

To update the data in the SQL table, the UPDATE statement is required. The basic syntax is: UPDATE table name SET column 1 = value 1, column 2 = value 2...WHERE conditions; 1. When updating a single row, specify a unique identifier, such as the primary key; 2. When updating multiple columns, list multiple columns and values ??in the SET clause; 3. When updating multiple rows, multiple records matching the WHERE conditions will be updated at the same time; 4. If the WHERE clause is omitted, all rows will be updated, so be careful to operate; 5. It is recommended to use transaction wrapping UPDATE operations to roll back when an error occurs; In addition, you should always use SELECT to test the conditions, backup important data, and use LIMIT in supported databases to limit the number of rows to be updated.

ThemedianinSQLiscalculatedusingROW_NUMBER()andCOUNT()orPERCENTILE_CONT(0.5).First,assignrownumbersandgettotalcountviawindowfunctions.Thendeterminemiddlepositions:ifcountisodd,pickthecentralvalue;ifeven,averagethetwomiddlevalues.UseaCTEtocomputepositi